为什么为 Docker 容器授予 SYS_ADMIN 权限“不好”？

Question

我遇到了安全团队的问题，因为工程团队想要在 Docker 中 FUSE 挂载文件系统，但是，要做到这一点，必须设置“--cap-add SYS_ADMIN”标志。安全不允许这个标志。

我在 Internet 上发现了很多关于 Docker 运行时期间的“--cap-add SYS_ADMIN”标志的文章，因为“SYS_ADMIN 本身授予了很大一部分功能，并且它可能会提供更多攻击面。”

但是，我找不到任何具体说明这些功能是什么以及它们呈现什么“攻击面”的内容？

SYS_ADMIN 标志究竟授予什么？

设置此标志会带来什么实际安全风险？

Answer 1

这基本上是对主机的 root 访问。从功能手册页：

CAP_SYS_ADMIN 注意：这个能力是重载的；请参阅下面的内核开发人员注意事项。

          * Perform a range of system administration operations
            including: quotactl(2), mount(2), umount(2), pivot_root(2),
            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), execve(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 not require 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)).