Linux capability详解
2024-05-24 02:15:36
前言
在Linux2.2前root权限简单划分为root和非root，那么root拥有全部权限，非root拥有有限的权限，如果非root用户需要安装软件，要么切换为root用户，要么使用sudo。如果我们以非root用户安装软件，会有如下提示，提示没权限。
aaa@ubuntu:/home/test$ apt-get install vim
E: Could not open lock file /var/lib/dpkg/lock-frontend - open (13: Permission denied)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), are you root?当我们使用setuid功能后，既让一个程序在运行时，能够获得root权限。如下所示
root@ubuntu:/home/test# chmod +s /usr/bin/apt-get
root@ubuntu:/home/test# ls -al /usr/bin/apt-get
-rwsr-sr-x 1 root root 47576 Jun 15 2021 /usr/bin/apt-get
root@ubuntu:/home/test# exit
exit
aaa@ubuntu:/home/test$ apt-get install vim
Reading package lists... Done
Building dependency tree
Reading state information... Done
vim is already the newest version (2:8.1.2269-1ubuntu5.9).
0 upgraded, 0 newly installed, 0 to remove and 126 not upgraded.非root用户成功执行apt-get安装软件。但是有没有发现，我们只需要安装功能，却在执行该进程时，拥有了root权限。可以做更多高危操作，在这种功能模式下，就是要么不给权限，要么给就给全部权限，新增了攻击面，那么接下来引入Capability，对此收敛，只需要给它需要的权限即可，收敛攻击面。
Linux capability
Linux capability是一种对于对 root 权限进行更细粒度的控制，实现按需授权，从而减小系统的安全攻击面的安全机制，用于补充对于root权限粒度控制不足的划分。
主要分为文件和进程capability。
进程Capability:
文件Capability:
| capability 名称 | 描述 | ||
|---|---|---|---|
| CAP_AUDIT_CONTROL | |||
| CAP_AUDIT_READ | 允许通过 multicast netlink 套接字读取审计日志 | ||
| CAP_AUDIT_WRITE | 将记录写入内核审计日志 | ||
| CAP_BLOCK_SUSPEND | 使用可以阻止系统挂起的特性 | ||
| CAP_CHOWN | 修改文件所有者的权限 | ||
| CAP_DAC_OVERRIDE | 忽略文件的 DAC 访问限制 | ||
| CAP_DAC_READ_SEARCH |
忽略文
件读及目录搜索的 DAC 访问限制 |
||
| CAP_FOWNER | 忽略文件属主 ID 必须和进程用户 ID 相匹配的限制 | ||
| CAP_FSETID | 允许设置文件的 setuid 位 | ||
| CAP_IPC_LOCK | 允许锁定共享内存片段 | ||
| CAP_IPC_OWNER | 忽略 IPC 所有权检查 | ||
| CAP_KILL | 允许对不属于自己的进程发送信号 | ||
| CAP_LEASE | 允许修改文件锁的 FL_LEASE 标志 | ||
| CAP_LINUX_IMMUTABLE | 允许修改文件的 IMMUTABLE 和 APPEND 属性标志 | ||
| CAP_MAC_ADMIN | 允许 MAC 配置或状态更改 | ||
| CAP_MAC_OVERRIDE | 覆盖 MAC(Mandatory Access Control) | ||
| CAP_MKNOD | 允许使用 mknod() 系统调用 | ||
| CAP_NET_ADMIN | 允许执行网络管理任务 | ||
| CAP_NET_BIND_SERVICE | 允许绑定到小于 1024 的端口 | ||
| CAP_NET_BROADCAST | 允许网络广播和多播访问 | ||
| CAP_NET_RAW | 允许使用原始套接字 | ||
| CAP_SETGID | 允许改变进程的 GID | ||
| CAP_SETFCAP | 允许为文件设置任意的 capabilities | ||
| CAP_SETPCAP | 参考 capabilities man page | ||
| CAP_SETUID | 允许改变进程的 UID | ||
| CAP_SYS_ADMIN | 允许执行系统管理任务，如加载或卸载文件系统、设置磁盘配额等 | ||
| CAP_SYS_BOOT | 允许重新启动系统 | ||
| CAP_SYS_CHROOT | 允许使用 chroot() 系统调用 | ||
| CAP_SYS_MODULE | 允许插入和删除内核模块 | ||
| CAP_SYS_NICE | 允许提升优先级及设置其他进程的优先级 | ||
| CAP_SYS_PACCT | 允许执行进程的 BSD 式审计 | ||
| CAP_SYS_PTRACE | 允许跟踪任何进程 | ||
| CAP_SYS_RAWIO | 允许直接访问 /devport、/dev/mem、/dev/kmem 及原始块设备 | ||
| CAP_SYS_RESOURCE | 忽略资源限制 | ||
| CAP_SYS_TIME | 允许改变系统时钟 | ||
| CAP_SYS_TTY_CONFIG | 允许配置 TTY 设备 | ||
| CAP_SYSLOG | 允许使用 syslog() 系统调用 | ||
| CAP_WAKE_ALARM | 允许触发一些能唤醒系统的东西(比如 CLOCK_BOOTTIME_ALARM 计时器) |
下列使用man capabilities
CAPABILITIES(7) Linux Programmer's Manual CAPABILITIES(7)NAMEcapabilities - overview of Linux capabilitiesDESCRIPTIONFor the purpose of performing permission checks, traditional UNIX implementations distinguish two categories of processes: privileged processes(whose effective user ID is 0, referred to as superuser or root), and unprivileged processes (whose effective UID is nonzero). Privileged pro‐cesses bypass all kernel permission checks, while unprivileged processes are subject to full permission checking based on the process's creden‐tials (usually: effective UID, effective GID, and supplementary group list).Starting with kernel 2.2, Linux divides the privileges traditionally associated with superuser into distinct units, known as capabilities, whichcan be independently enabled and disabled. Capabilities are a per-thread attribute.Capabilities listThe following list shows the capabilities implemented on Linux, and the operations or behaviors that each capability permits:CAP_AUDIT_CONTROL (since Linux 2.6.11)Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.CAP_AUDIT_READ (since Linux 3.16)Allow reading the audit log via a multicast netlink socket.CAP_AUDIT_WRITE (since Linux 2.6.11)Write records to kernel auditing log.CAP_BLOCK_SUSPEND (since Linux 3.5)Employ features that can block system suspend (epoll(7) EPOLLWAKEUP, /proc/sys/wake_lock).CAP_CHOWNMake arbitrary changes to file UIDs and GIDs (see chown(2)).CAP_DAC_OVERRIDEBypass file read, write, and execute permission checks. (DAC is an abbreviation of "discretionary access control".)CAP_DAC_READ_SEARCH* Bypass file read permission checks and directory read and execute permission checks;* invoke open_by_handle_at(2);* use the linkat(2) AT_EMPTY_PATH flag to create a link to a file referred to by a file descriptor.CAP_FOWNER* Bypass permission checks on operations that normally require the filesystem UID of the process to match the UID of the file (e.g.,chmod(2), utime(2)), excluding those operations covered by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;* set inode flags (see ioctl_iflags(2)) on arbitrary files;* set Access Control Lists (ACLs) on arbitrary files;* ignore directory sticky bit on file deletion;* modify user extended attributes on sticky directory owned by any user;* specify O_NOATIME for arbitrary files in open(2) and fcntl(2).CAP_FSETID* Don't clear set-user-ID and set-group-ID mode bits when a file is modified;* set the set-group-ID bit for a file whose GID does not match the filesystem or any of the supplementary GIDs of the calling process.CAP_IPC_LOCKLock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).CAP_IPC_OWNERBypass permission checks for operations on System V IPC objects.CAP_KILLBypass permission checks for sending signals (see kill(2)). This includes use of the ioctl(2) KDSIGACCEPT operation.CAP_LEASE (since Linux 2.4)Establish leases on arbitrary files (see fcntl(2)).CAP_LINUX_IMMUTABLESet the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see ioctl_iflags(2)).CAP_MAC_ADMIN (since Linux 2.6.25)Allow MAC configuration or state changes. Implemented for the Smack Linux Security Module (LSM).CAP_MAC_OVERRIDE (since Linux 2.6.25)Override Mandatory Access Control (MAC). Implemented for the Smack LSM.CAP_MKNOD (since Linux 2.4)Create special files using mknod(2).CAP_NET_ADMINPerform various network-related operations:* interface configuration;* administration of IP firewall, masquerading, and accounting;* modify routing tables;* bind to any address for transparent proxying;* set type-of-service (TOS)* clear driver statistics;* set promiscuous mode;* enabling multicasting;* use setsockopt(2) to set the following socket options: SO_DEBUG, SO_MARK, SO_PRIORITY (for a priority outside the range 0 to 6),SO_RCVBUFFORCE, and SO_SNDBUFFORCE.CAP_NET_BIND_SERVICEBind a socket to Internet domain privileged ports (port numbers less than 1024).CAP_NET_BROADCAST(Unused) Make socket broadcasts, and listen to multicasts.CAP_NET_RAW* Use RAW and PACKET sockets;* bind to any address for transparent proxying.CAP_SETGID* Make arbitrary manipulations of process GIDs and supplementary GID list;* forge GID when passing socket credentials via UNIX domain sockets;* write a group ID mapping in a user namespace (see user_namespaces(7)).CAP_SETFCAP (since Linux 2.6.24)Set arbitrary capabilities on a file.CAP_SETPCAPIf file capabilities are supported (i.e., since Linux 2.6.24): add any capability from the calling thread's bounding set to its inherita‐ble set; drop capabilities from the bounding set (via prctl(2) PR_CAPBSET_DROP); make changes to the securebits flags.If file capabilities are not supported (i.e., kernels before Linux 2.6.24): grant or remove any capability in the caller's permitted ca‐pability set to or from any other process. (This property of CAP_SETPCAP is not available when the kernel is configured to support filecapabilities, since CAP_SETPCAP has entirely different semantics for such kernels.)CAP_SETUID* Make arbitrary manipulations of process UIDs (setuid(2), setreuid(2), setresuid(2), setfsuid(2));* forge UID when passing socket credentials via UNIX domain sockets;* write a user ID mapping in a user namespace (see user_namespaces(7)).CAP_SYS_ADMINNote: this capability is overloaded; see Notes to kernel developers, below.* Perform a range of system administration operations including: quotactl(2), mount(2), umount(2), pivot_root(2), swapon(2), swapoff(2),sethostname(2), and setdomainname(2);* perform privileged syslog(2) operations (since Linux 2.6.37, CAP_SYSLOG should be used to permit such operations);* perform VM86_REQUEST_IRQ vm86(2) command;* perform IPC_SET and IPC_RMID operations on arbitrary System V IPC objects;* override RLIMIT_NPROC resource limit;* perform operations on trusted and security Extended Attributes (see xattr(7));* use lookup_dcookie(2);* use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux 2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;* forge PID when passing socket credentials via UNIX domain sockets;* exceed /proc/sys/fs/file-max, the system-wide limit on the number of open files, in system calls that open files (e.g., accept(2), ex‐ecve(2), open(2), pipe(2));* employ CLONE_* flags that create new namespaces with clone(2) and unshare(2) (but, since Linux 3.8, creating user namespaces does notrequire any capability);* call perf_event_open(2);* access privileged perf event information;* call setns(2) (requires CAP_SYS_ADMIN in the target namespace);* call fanotify_init(2);* call bpf(2);* perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;* perform madvise(2) MADV_HWPOISON operation;* employ the TIOCSTI ioctl(2) to insert characters into the input queue of a terminal other than the caller's controlling terminal;* employ the obsolete nfsservctl(2) system call;* employ the obsolete bdflush(2) system call;* perform various privileged block-device ioctl(2) operations;* perform various privileged filesystem ioctl(2) operations;* perform privileged ioctl(2) operations on the /dev/random device (see random(4));* install a seccomp(2) filter without first having to set the no_new_privs thread attribute;* modify allow/deny rules for device control groups;* employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation to dump tracee's seccomp filters;* employ the ptrace(2) PTRACE_SETOPTIONS operation to suspend the tracee's seccomp protections (i.e., the PTRACE_O_SUSPEND_SECCOMP flag);* perform administrative operations on many device drivers.* Modify autogroup nice values by writing to /proc/[pid]/autogroup (see sched(7)).CAP_SYS_BOOTUse reboot(2) and kexec_load(2).CAP_SYS_CHROOT* Use chroot(2);* change mount namespaces using setns(2).CAP_SYS_MODULE* Load and unload kernel modules (see init_module(2) and delete_module(2));* in kernels before 2.6.25: drop capabilities from the system-wide capability bounding set.CAP_SYS_NICE* Raise process nice value (nice(2), setpriority(2)) and change the nice value for arbitrary processes;* set real-time scheduling policies for calling process, and set scheduling policies and priorities for arbitrary processes(sched_setscheduler(2), sched_setparam(2), sched_setattr(2));* set CPU affinity for arbitrary processes (sched_setaffinity(2));* set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));* apply migrate_pages(2) to arbitrary processes and allow processes to be migrated to arbitrary nodes;* apply move_pages(2) to arbitrary processes;* use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).CAP_SYS_PACCTUse acct(2).CAP_SYS_PTRACE* Trace arbitrary processes using ptrace(2);* apply get_robust_list(2) to arbitrary processes;* transfer data to or from the memory of arbitrary processes using process_vm_readv(2) and process_vm_writev(2);* inspect processes using kcmp(2).CAP_SYS_RAWIO* Perform I/O port operations (iopl(2) and ioperm(2));* access /proc/kcore;* employ the FIBMAP ioctl(2) operation;* open devices for accessing x86 model-specific registers (MSRs, see msr(4));* update /proc/sys/vm/mmap_min_addr;* create memory mappings at addresses below the value specified by /proc/sys/vm/mmap_min_addr;* map files in /proc/bus/pci;* open /dev/mem and /dev/kmem;* perform various SCSI device commands;* perform certain operations on hpsa(4) and cciss(4) devices;* perform a range of device-specific operations on other devices.CAP_SYS_RESOURCE* Use reserved space on ext2 filesystems;* make ioctl(2) calls controlling ext3 journaling;* override disk quota limits;* increase resource limits (see setrlimit(2));* override RLIMIT_NPROC resource limit;* override maximum number of consoles on console allocation;* override maximum number of keymaps;* allow more than 64hz interrupts from the real-time clock;* raise msg_qbytes limit for a System V message queue above the limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));* allow the RLIMIT_NOFILE resource limit on the number of "in-flight" file descriptors to be bypassed when passing file descriptors toanother process via a UNIX domain socket (see unix(7));* override the /proc/sys/fs/pipe-size-max limit when setting the capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.* use F_SETPIPE_SZ to increase the capacity of a pipe above the limit specified by /proc/sys/fs/pipe-max-size;* override /proc/sys/fs/mqueue/queues_max limit when creating POSIX message queues (see mq_overview(7));* employ the prctl(2) PR_SET_MM operation;* set /proc/[pid]/oom_score_adj to a value lower than the value last set by a process with CAP_SYS_RESOURCE.CAP_SYS_TIMESet system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware) clock.CAP_SYS_TTY_CONFIGUse vhangup(2); employ various privileged ioctl(2) operations on virtual terminals.CAP_SYSLOG (since Linux 2.6.37)* Perform privileged syslog(2) operations. See syslog(2) for information on which operations require privilege.* View kernel addresses exposed via /proc and other interfaces when /proc/sys/kernel/kptr_restrict has the value 1. (See the discussionof the kptr_restrict in proc(5).)CAP_WAKE_ALARM (since Linux 3.0)Trigger something that will wake up the system (set CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM timers).Past and current implementationA full implementation of capabilities requires that:1. For all privileged operations, the kernel must check whether the thread has the required capability in its effective set.2. The kernel must provide system calls allowing a thread's capability sets to be changed and retrieved.3. The filesystem must support attaching capabilities to an executable file, so that a process gains those capabilities when the file is exe‐cuted.Before kernel 2.6.24, only the first two of these requirements are met; since kernel 2.6.24, all three requirements are met.Notes to kernel developersWhen adding a new kernel feature that should be governed by a capability, consider the following points.* The goal of capabilities is divide the power of superuser into pieces, such that if a program that has one or more capabilities is compro‐mised, its power to do damage to the system would be less than the same program running with root privilege.* You have the choice of either creating a new capability for your new feature, or associating the feature with one of the existing capabili‐ties. In order to keep the set of capabilities to a manageable size, the latter option is preferable, unless there are compelling reasons totake the former option. (There is also a technical limit: the size of capability sets is currently limited to 64 bits.)* To determine which existing capability might best be associated with your new feature, review the list of capabilities above in order to finda "silo" into which your new feature best fits. One approach to take is to determine if there are other features requiring capabilities thatwill always be used along with the new feature. If the new feature is useless without these other features, you should use the same capabil‐ity as the other features.* Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A vast proportion of existing capability checks are associated with this capability(see the partial list above). It can plausibly be called "the new root", since on the one hand, it confers a wide range of powers, and onthe other hand, its broad scope means that this is the capability that is required by many privileged programs. Don't make the problemworse. The only new features that should be associated with CAP_SYS_ADMIN are ones that closely match existing uses in that silo.* If you have determined that it really is necessary to create a new capability for your feature, don't make or name it as a "single-use" capa‐bility. Thus, for example, the addition of the highly specific CAP_SYS_PACCT was probably a mistake. Instead, try to identify and name yournew capability as a broader silo into which other related future use cases might fit.Thread capability setsEach thread has the following capability sets containing zero or more of the above capabilities:PermittedThis is a limiting superset for the effective capabilities that the thread may assume. It is also a limiting superset for the capabili‐ties that may be added to the inheritable set by a thread that does not have the CAP_SETPCAP capability in its effective set.If a thread drops a capability from its permitted set, it can never reacquire that capability (unless it execve(2)s either a set-user-ID-root program, or a program whose associated file capabilities grant that capability).InheritableThis is a set of capabilities preserved across an execve(2). Inheritable capabilities remain inheritable when executing any program, andinheritable capabilities are added to the permitted set when executing a program that has the corresponding bits set in the file inheri‐table set.Because inheritable capabilities are not generally preserved across execve(2) when running as a non-root user, applications that wish torun helper programs with elevated capabilities should consider using ambient capabilities, described below.EffectiveThis is the set of capabilities used by the kernel to perform permission checks for the thread.Bounding (per-thread since Linux 2.6.25)The capability bounding set is a mechanism that can be used to limit the capabilities that are gained during execve(2).Since Linux 2.6.25, this is a per-thread capability set. In older kernels, the capability bounding set was a system wide attributeshared by all threads on the system.For more details on the capability bounding set, see below.Ambient (since Linux 4.3)This is a set of capabilities that are preserved across an execve(2) of a program that is not privileged. The ambient capability setobeys the invariant that no capability can ever be ambient if it is not both permitted and inheritable.The ambient capability set can be directly modified using prctl(2). Ambient capabilities are automatically lowered if either of the cor‐responding permitted or inheritable capabilities is lowered.Executing a program that changes UID or GID due to the set-user-ID or set-group-ID bits or executing a program that has any file capabil‐ities set will clear the ambient set. Ambient capabilities are added to the permitted set and assigned to the effective set when ex‐ecve(2) is called. If ambient capabilities cause a process's permitted and effective capabilities to increase during an execve(2), thisdoes not trigger the secure-execution mode described in ld.so(8).A child created via fork(2) inherits copies of its parent's capability sets. See below for a discussion of the treatment of capabilities duringexecve(2).Using capset(2), a thread may manipulate its own capability sets (see below).Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the numerical value of the highest capability supported by the running kernel;this can be used to determine the highest bit that may be set in a capability set.File capabilitiesSince kernel 2.6.24, the kernel supports associating capability sets with an executable file using setcap(8). The file capability sets arestored in an extended attribute (see setxattr(2) and xattr(7)) named security.capability. Writing to this extended attribute requires theCAP_SETFCAP capability. The file capability sets, in conjunction with the capability sets of the thread, determine the capabilities of a threadafter an execve(2).The three file capability sets are:Permitted (formerly known as forced):These capabilities are automatically permitted to the thread, regardless of the thread's inheritable capabilities.Inheritable (formerly known as allowed):This set is ANDed with the thread's inheritable set to determine which inheritable capabilities are enabled in the permitted set of thethread after the execve(2).Effective:This is not a set, but rather just a single bit. If this bit is set, then during an execve(2) all of the new permitted capabilities forthe thread are also raised in the effective set. If this bit is not set, then after an execve(2), none of the new permitted capabilitiesis in the new effective set.Enabling the file effective capability bit implies that any file permitted or inheritable capability that causes a thread to acquire thecorresponding permitted capability during an execve(2) (see the transformation rules described below) will also acquire that capabilityin its effective set. Therefore, when assigning capabilities to a file (setcap(8), cap_set_file(3), cap_set_fd(3)), if we specify theeffective flag as being enabled for any capability, then the effective flag must also be specified as enabled for all other capabilitiesfor which the corresponding permitted or inheritable flags is enabled.File capability extended attribute versioningTo allow extensibility, the kernel supports a scheme to encode a version number inside the security.capability extended attribute that is usedto implement file capabilities. These version numbers are internal to the implementation, and not directly visible to user-space applications.To date, the following versions are supported:VFS_CAP_REVISION_1This was the original file capability implementation, which supported 32-bit masks for file capabilities.VFS_CAP_REVISION_2 (since Linux 2.6.25)This version allows for file capability masks that are 64 bits in size, and was necessary as the number of supported capabilities grewbeyond 32. The kernel transparently continues to support the execution of files that have 32-bit version 1 capability masks, but whenadding capabilities to files that did not previously have capabilities, or modifying the capabilities of existing files, it automaticallyuses the version 2 scheme (or possibly the version 3 scheme, as described below).VFS_CAP_REVISION_3 (since Linux 4.14)Version 3 file capabilities are provided to support namespaced file capabilities (described below).As with version 2 file capabilities, version 3 capability masks are 64 bits in size. But in addition, the root user ID of namespace isencoded in the security.capability extended attribute. (A namespace's root user ID is the value that user ID 0 inside that namespacemaps to in the initial user namespace.)Version 3 file capabilities are designed to coexist with version 2 capabilities; that is, on a modern Linux system, there may be somefiles with version 2 capabilities while others have version 3 capabilities.Before Linux 4.14, the only kind of file capability extended attribute that could be attached to a file was a VFS_CAP_REVISION_2 attribute.Since Linux 4.14, the version of the security.capability extended attribute that is attached to a file depends on the circumstances in which theattribute was created.Starting with Linux 4.14, a security.capability extended attribute is automatically created as (or converted to) a version 3 (VFS_CAP_REVI‐SION_3) attribute if both of the following are true:(1) The thread writing the attribute resides in a noninitial user namespace. (More precisely: the thread resides in a user namespace other thanthe one from which the underlying filesystem was mounted.)(2) The thread has the CAP_SETFCAP capability over the file inode, meaning that (a) the thread has the CAP_SETFCAP capability in its own usernamespace; and (b) the UID and GID of the file inode have mappings in the writer's user namespace.When a VFS_CAP_REVISION_3 security.capability extended attribute is created, the root user ID of the creating thread's user namespace is savedin the extended attribute.By contrast, creating or modifying a security.capability extended attribute from a privileged (CAP_SETFCAP) thread that resides in the namespacewhere the underlying filesystem was mounted (this normally means the initial user namespace) automatically results in the creation of a version2 (VFS_CAP_REVISION_2) attribute.Note that the creation of a version 3 security.capability extended attribute is automatic. That is to say, when a user-space application writes(setxattr(2)) a security.capability attribute in the version 2 format, the kernel will automatically create a version 3 attribute if the attri‐bute is created in the circumstances described above. Correspondingly, when a version 3 security.capability attribute is retrieved (getx‐attr(2)) by a process that resides inside a user namespace that was created by the root user ID (or a descendant of that user namespace), thereturned attribute is (automatically) simplified to appear as a version 2 attribute (i.e., the returned value is the size of a version 2 attri‐bute and does not include the root user ID). These automatic translations mean that no changes are required to user-space tools (e.g., set‐cap(1) and getcap(1)) in order for those tools to be used to create and retrieve version 3 security.capability attributes.Note that a file can have either a version 2 or a version 3 security.capability extended attribute associated with it, but not both: creation ormodification of the security.capability extended attribute will automatically modify the version according to the circumstances in which the ex‐tended attribute is created or modified.Transformation of capabilities during execve()During an execve(2), the kernel calculates the new capabilities of the process using the following algorithm:P'(ambient) = (file is privileged) ? 0 : P(ambient)P'(permitted) = (P(inheritable) & F(inheritable)) |(F(permitted) & P(bounding)) | P'(ambient)P'(effective) = F(effective) ? P'(permitted) : P'(ambient)P'(inheritable) = P(inheritable) [i.e., unchanged]P'(bounding) = P(bounding) [i.e., unchanged]where:P() denotes the value of a thread capability set before the execve(2)P'() denotes the value of a thread capability set after the execve(2)F() denotes a file capability setNote the following details relating to the above capability transformation rules:* The ambient capability set is present only since Linux 4.3. When determining the transformation of the ambient set during execve(2), a priv‐ileged file is one that has capabilities or has the set-user-ID or set-group-ID bit set.* Prior to Linux 2.6.25, the bounding set was a system-wide attribute shared by all threads. That system-wide value was employed to calculatethe new permitted set during execve(2) in the same manner as shown above for P(bounding).Note: during the capability transitions described above, file capabilities may be ignored (treated as empty) for the same reasons that the set-user-ID and set-group-ID bits are ignored; see execve(2). File capabilities are similarly ignored if the kernel was booted with theno_file_caps option.Note: according to the rules above, if a process with nonzero user IDs performs an execve(2) then any capabilities that are present in its per‐mitted and effective sets will be cleared. For the treatment of capabilities when a process with a user ID of zero performs an execve(2), seebelow under Capabilities and execution of programs by root.Safety checking for capability-dumb binariesA capability-dumb binary is an application that has been marked to have file capabilities, but has not been converted to use the libcap(3) APIto manipulate its capabilities. (In other words, this is a traditional set-user-ID-root program that has been switched to use file capabili‐ties, but whose code has not been modified to understand capabilities.) For such applications, the effective capability bit is set on the file,so that the file permitted capabilities are automatically enabled in the process effective set when executing the file. The kernel recognizes afile which has the effective capability bit set as capability-dumb for the purpose of the check described here.When executing a capability-dumb binary, the kernel checks if the process obtained all permitted capabilities that were specified in the filepermitted set, after the capability transformations described above have been performed. (The typical reason why this might not occur is thatthe capability bounding set masked out some of the capabilities in the file permitted set.) If the process did not obtain the full set of filepermitted capabilities, then execve(2) fails with the error EPERM. This prevents possible security risks that could arise when a capability-dumb application is executed with less privilege that it needs. Note that, by definition, the application could not itself recognize this prob‐lem, since it does not employ the libcap(3) API.Capabilities and execution of programs by rootIn order to mirror traditional UNIX semantics, the kernel performs special treatment of file capabilities when a process with UID 0 (root) exe‐cutes a program and when a set-user-ID-root program is executed.After having performed any changes to the process effective ID that were triggered by the set-user-ID mode bit of the binary—e.g., switching theeffective user ID to 0 (root) because a set-user-ID-root program was executed—the kernel calculates the file capability sets as follows:1. If the real or effective user ID of the process is 0 (root), then the file inheritable and permitted sets are ignored; instead they are no‐tionally considered to be all ones (i.e., all capabilities enabled). (There is one exception to this behavior, described below in Set-user-ID-root programs that have file capabilities.)2. If the effective user ID of the process is 0 (root) or the file effective bit is in fact enabled, then the file effective bit is notionallydefined to be one (enabled).These notional values for the file's capability sets are then used as described above to calculate the transformation of the process's capabili‐ties during execve(2).Thus, when a process with nonzero UIDs execve(2)s a set-user-ID-root program that does not have capabilities attached, or when a process whosereal and effective UIDs are zero execve(2)s a program, the calculation of the process's new permitted capabilities simplifies to:P'(permitted) = P(inheritable) | P(bounding)P'(effective) = P'(permitted)Consequently, the process gains all capabilities in its permitted and effective capability sets, except those masked out by the capabilitybounding set. (In the calculation of P'(permitted), the P'(ambient) term can be simplified away because it is by definition a proper subset ofP(inheritable).)The special treatments of user ID 0 (root) described in this subsection can be disabled using the securebits mechanism described below.Set-user-ID-root programs that have file capabilitiesThere is one exception to the behavior described under Capabilities and execution of programs by root. If (a) the binary that is being executedhas capabilities attached and (b) the real user ID of the process is not 0 (root) and (c) the effective user ID of the process is 0 (root), thenthe file capability bits are honored (i.e., they are not notionally considered to be all ones). The usual way in which this situation can ariseis when executing a set-UID-root program that also has file capabilities. When such a program is executed, the process gains just the capabili‐ties granted by the program (i.e., not all capabilities, as would occur when executing a set-user-ID-root program that does not have any associ‐ated file capabilities).Note that one can assign empty capability sets to a program file, and thus it is possible to create a set-user-ID-root program that changes theeffective and saved set-user-ID of the process that executes the program to 0, but confers no capabilities to that process.Capability bounding setThe capability bounding set is a security mechanism that can be used to limit the capabilities that can be gained during an execve(2). Thebounding set is used in the following ways:* During an execve(2), the capability bounding set is ANDed with the file permitted capability set, and the result of this operation is assignedto the thread's permitted capability set. The capability bounding set thus places a limit on the permitted capabilities that may be grantedby an executable file.* (Since Linux 2.6.25) The capability bounding set acts as a limiting superset for the capabilities that a thread can add to its inheritable setusing capset(2). This means that if a capability is not in the bounding set, then a thread can't add this capability to its inheritable set,even if it was in its permitted capabilities, and thereby cannot have this capability preserved in its permitted set when it execve(2)s a filethat has the capability in its inheritable set.Note that the bounding set masks the file permitted capabilities, but not the inheritable capabilities. If a thread maintains a capability inits inheritable set that is not in its bounding set, then it can still gain that capability in its permitted set by executing a file that hasthe capability in its inheritable set.Depending on the kernel version, the capability bounding set is either a system-wide attribute, or a per-process attribute.Capability bounding set from Linux 2.6.25 onwardFrom Linux 2.6.25, the capability bounding set is a per-thread attribute. (The system-wide capability bounding set described below no longerexists.)The bounding set is inherited at fork(2) from the thread's parent, and is preserved across an execve(2).A thread may remove capabilities from its capability bounding set using the prctl(2) PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAPcapability. Once a capability has been dropped from the bounding set, it cannot be restored to that set. A thread can determine if a capabil‐ity is in its bounding set using the prctl(2) PR_CAPBSET_READ operation.Removing capabilities from the bounding set is supported only if file capabilities are compiled into the kernel. In kernels before Linux2.6.33, file capabilities were an optional feature configurable via the CONFIG_SECURITY_FILE_CAPABILITIES option. Since Linux 2.6.33, the con‐figuration option has been removed and file capabilities are always part of the kernel. When file capabilities are compiled into the kernel,the init process (the ancestor of all processes) begins with a full bounding set. If file capabilities are not compiled into the kernel, theninit begins with a full bounding set minus CAP_SETPCAP, because this capability has a different meaning when there are no file capabilities.Removing a capability from the bounding set does not remove it from the thread's inheritable set. However it does prevent the capability frombeing added back into the thread's inheritable set in the future.Capability bounding set prior to Linux 2.6.25In kernels before 2.6.25, the capability bounding set is a system-wide attribute that affects all threads on the system. The bounding set isaccessible via the file /proc/sys/kernel/cap-bound. (Confusingly, this bit mask parameter is expressed as a signed decimal number in/proc/sys/kernel/cap-bound.)Only the init process may set capabilities in the capability bounding set; other than that, the superuser (more precisely: a process with theCAP_SYS_MODULE capability) may only clear capabilities from this set.On a standard system the capability bounding set always masks out the CAP_SETPCAP capability. To remove this restriction (dangerous!), modifythe definition of CAP_INIT_EFF_SET in include/linux/capability.h and rebuild the kernel.The system-wide capability bounding set feature was added to Linux starting with kernel version 2.2.11.Effect of user ID changes on capabilitiesTo preserve the traditional semantics for transitions between 0 and nonzero user IDs, the kernel makes the following changes to a thread's capa‐bility sets on changes to the thread's real, effective, saved set, and filesystem user IDs (using setuid(2), setresuid(2), or similar):1. If one or more of the real, effective or saved set user IDs was previously 0, and as a result of the UID changes all of these IDs have a non‐zero value, then all capabilities are cleared from the permitted, effective, and ambient capability sets.2. If the effective user ID is changed from 0 to nonzero, then all capabilities are cleared from the effective set.3. If the effective user ID is changed from nonzero to 0, then the permitted set is copied to the effective set.4. If the filesystem user ID is changed from 0 to nonzero (see setfsuid(2)), then the following capabilities are cleared from the effective set:CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.6.30), CAP_MAC_OVERRIDE, andCAP_MKNOD (since Linux 2.6.30). If the filesystem UID is changed from nonzero to 0, then any of these capabilities that are enabled in thepermitted set are enabled in the effective set.If a thread that has a 0 value for one or more of its user IDs wants to prevent its permitted capability set being cleared when it resets all ofits user IDs to nonzero values, it can do so using the SECBIT_KEEP_CAPS securebits flag described below.Programmatically adjusting capability setsA thread can retrieve and change its permitted, effective, and inheritable capability sets using the capget(2) and capset(2) system calls. How‐ever, the use of cap_get_proc(3) and cap_set_proc(3), both provided in the libcap package, is preferred for this purpose. The following rulesgovern changes to the thread capability sets:1. If the caller does not have the CAP_SETPCAP capability, the new inheritable set must be a subset of the combination of the existing inherita‐ble and permitted sets.2. (Since Linux 2.6.25) The new inheritable set must be a subset of the combination of the existing inheritable set and the capability boundingset.3. The new permitted set must be a subset of the existing permitted set (i.e., it is not possible to acquire permitted capabilities that thethread does not currently have).4. The new effective set must be a subset of the new permitted set.The securebits flags: establishing a capabilities-only environmentStarting with kernel 2.6.26, and with a kernel in which file capabilities are enabled, Linux implements a set of per-thread securebits flagsthat can be used to disable special handling of capabilities for UID 0 (root). These flags are as follows:SECBIT_KEEP_CAPSSetting this flag allows a thread that has one or more 0 UIDs to retain capabilities in its permitted set when it switches all of itsUIDs to nonzero values. If this flag is not set, then such a UID switch causes the thread to lose all permitted capabilities. This flagis always cleared on an execve(2).Note that even with the SECBIT_KEEP_CAPS flag set, the effective capabilities of a thread are cleared when it switches its effective UIDto a nonzero value. However, if the thread has set this flag and its effective UID is already nonzero, and the thread subsequentlyswitches all other UIDs to nonzero values, then the effective capabilities will not be cleared.The setting of the SECBIT_KEEP_CAPS flag is ignored if the SECBIT_NO_SETUID_FIXUP flag is set. (The latter flag provides a superset ofthe effect of the former flag.)This flag provides the same functionality as the older prctl(2) PR_SET_KEEPCAPS operation.SECBIT_NO_SETUID_FIXUPSetting this flag stops the kernel from adjusting the process's permitted, effective, and ambient capability sets when the thread's ef‐fective and filesystem UIDs are switched between zero and nonzero values. (See the subsection Effect of user ID changes on capabili‐ties.)SECBIT_NOROOTIf this bit is set, then the kernel does not grant capabilities when a set-user-ID-root program is executed, or when a process with aneffective or real UID of 0 calls execve(2). (See the subsection Capabilities and execution of programs by root.)SECBIT_NO_CAP_AMBIENT_RAISESetting this flag disallows raising ambient capabilities via the prctl(2) PR_CAP_AMBIENT_RAISE operation.Each of the above "base" flags has a companion "locked" flag. Setting any of the "locked" flags is irreversible, and has the effect of prevent‐ing further changes to the corresponding "base" flag. The locked flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED, SECBIT_NO‐ROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.The securebits flags can be modified and retrieved using the prctl(2) PR_SET_SECUREBITS and PR_GET_SECUREBITS operations. The CAP_SETPCAP capa‐bility is required to modify the flags. Note that the SECBIT_* constants are available only after including the <linux/securebits.h> headerfile.The securebits flags are inherited by child processes. During an execve(2), all of the flags are preserved, except SECBIT_KEEP_CAPS which isalways cleared.An application can use the following call to lock itself, and all of its descendants, into an environment where the only way of gaining capabil‐ities is by executing a program with associated file capabilities:prctl(PR_SET_SECUREBITS,/* SECBIT_KEEP_CAPS off */SECBIT_KEEP_CAPS_LOCKED |SECBIT_NO_SETUID_FIXUP |SECBIT_NO_SETUID_FIXUP_LOCKED |SECBIT_NOROOT |SECBIT_NOROOT_LOCKED);/* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISEis not required */Per-user-namespace "set-user-ID-root" programsA set-user-ID program whose UID matches the UID that created a user namespace will confer capabilities in the process's permitted and effectivesets when executed by any process inside that namespace or any descendant user namespace.The rules about the transformation of the process's capabilities during the execve(2) are exactly as described in the subsections Transformationof capabilities during execve() and Capabilities and execution of programs by root, with the difference that, in the latter subsection, "root"is the UID of the creator of the user namespace.Namespaced file capabilitiesTraditional (i.e., version 2) file capabilities associate only a set of capability masks with a binary executable file. When a process executesa binary with such capabilities, it gains the associated capabilities (within its user namespace) as per the rules described above in "Transfor‐mation of capabilities during execve()".Because version 2 file capabilities confer capabilities to the executing process regardless of which user namespace it resides in, only privi‐leged processes are permitted to associate capabilities with a file. Here, "privileged" means a process that has the CAP_SETFCAP capability inthe user namespace where the filesystem was mounted (normally the initial user namespace). This limitation renders file capabilities uselessfor certain use cases. For example, in user-namespaced containers, it can be desirable to be able to create a binary that confers capabilitiesonly to processes executed inside that container, but not to processes that are executed outside the container.Linux 4.14 added so-called namespaced file capabilities to support such use cases. Namespaced file capabilities are recorded as version 3(i.e., VFS_CAP_REVISION_3) security.capability extended attributes. Such an attribute is automatically created in the circumstances describedabove under "File capability extended attribute versioning". When a version 3 security.capability extended attribute is created, the kernelrecords not just the capability masks in the extended attribute, but also the namespace root user ID.As with a binary that has VFS_CAP_REVISION_2 file capabilities, a binary with VFS_CAP_REVISION_3 file capabilities confers capabilities to aprocess during execve(). However, capabilities are conferred only if the binary is executed by a process that resides in a user namespace whoseUID 0 maps to the root user ID that is saved in the extended attribute, or when executed by a process that resides in a descendant of such anamespace.Interaction with user namespacesFor further information on the interaction of capabilities and user namespaces, see user_namespaces(7).CONFORMING TONo standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see⟨https://archive.org/details/posix_1003.1e-990310⟩.NOTESWhen attempting to strace(1) binaries that have capabilities (or set-user-ID-root binaries), you may find the -u <username> option useful.Something like:$ sudo strace -o trace.log -u ceci ./myprivprogFrom kernel 2.5.27 to kernel 2.6.26, capabilities were an optional kernel component, and could be enabled/disabled via the CONFIG_SECURITY_CAPA‐BILITIES kernel configuration option.The /proc/[pid]/task/TID/status file can be used to view the capability sets of a thread. The /proc/[pid]/status file shows the capability setsof a process's main thread. Before Linux 3.8, nonexistent capabilities were shown as being enabled (1) in these sets. Since Linux 3.8, allnonexistent capabilities (above CAP_LAST_CAP) are shown as disabled (0).The libcap package provides a suite of routines for setting and getting capabilities that is more comfortable and less likely to change than theinterface provided by capset(2) and capget(2). This package also provides the setcap(8) and getcap(8) programs. It can be found at⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/⟩.Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file capabilities are not enabled, a thread with the CAP_SETPCAP capability canmanipulate the capabilities of threads other than itself. However, this is only theoretically possible, since no thread ever has CAP_SETPCAP ineither of these cases:* In the pre-2.6.25 implementation the system-wide capability bounding set, /proc/sys/kernel/cap-bound, always masks out the CAP_SETPCAP capa‐bility, and this can not be changed without modifying the kernel source and rebuilding the kernel.* If file capabilities are disabled (i.e., the kernel CONFIG_SECURITY_FILE_CAPABILITIES option is disabled), then init starts out with theCAP_SETPCAP capability removed from its per-process bounding set, and that bounding set is inherited by all other processes created on thesystem.SEE ALSOcapsh(1), setpriv(1), prctl(2), setfsuid(2), cap_clear(3), cap_copy_ext(3), cap_from_text(3), cap_get_file(3), cap_get_proc(3), cap_init(3),capgetp(3), capsetp(3), libcap(3), proc(5), credentials(7), pthreads(7), user_namespaces(7), captest(8), filecap(8), getcap(8), netcap(8),pscap(8), setcap(8)include/linux/capability.h in the Linux kernel source treeCOLOPHONThis page is part of release 5.05 of the Linux man-pages project. A description of the project, information about reporting bugs, and the lat‐est version of this page, can be found at https://www.kernel.org/doc/man-pages/.Linux 2019-08-02 CAPABILITIES(7)Linux capability详解相关推荐
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