58 lines
2.7 KiB
Plaintext
58 lines
2.7 KiB
Plaintext
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The execve system call can grant a newly-started program privileges that
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its parent did not have. The most obvious examples are setuid/setgid
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programs and file capabilities. To prevent the parent program from
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gaining these privileges as well, the kernel and user code must be
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careful to prevent the parent from doing anything that could subvert the
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child. For example:
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- The dynamic loader handles LD_* environment variables differently if
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a program is setuid.
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- chroot is disallowed to unprivileged processes, since it would allow
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/etc/passwd to be replaced from the point of view of a process that
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inherited chroot.
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- The exec code has special handling for ptrace.
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These are all ad-hoc fixes. The no_new_privs bit (since Linux 3.5) is a
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new, generic mechanism to make it safe for a process to modify its
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execution environment in a manner that persists across execve. Any task
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can set no_new_privs. Once the bit is set, it is inherited across fork,
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clone, and execve and cannot be unset. With no_new_privs set, execve
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promises not to grant the privilege to do anything that could not have
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been done without the execve call. For example, the setuid and setgid
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bits will no longer change the uid or gid; file capabilities will not
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add to the permitted set, and LSMs will not relax constraints after
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execve.
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To set no_new_privs, use prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0).
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Be careful, though: LSMs might also not tighten constraints on exec
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in no_new_privs mode. (This means that setting up a general-purpose
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service launcher to set no_new_privs before execing daemons may
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interfere with LSM-based sandboxing.)
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Note that no_new_privs does not prevent privilege changes that do not
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involve execve. An appropriately privileged task can still call
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setuid(2) and receive SCM_RIGHTS datagrams.
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There are two main use cases for no_new_privs so far:
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- Filters installed for the seccomp mode 2 sandbox persist across
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execve and can change the behavior of newly-executed programs.
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Unprivileged users are therefore only allowed to install such filters
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if no_new_privs is set.
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- By itself, no_new_privs can be used to reduce the attack surface
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available to an unprivileged user. If everything running with a
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given uid has no_new_privs set, then that uid will be unable to
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escalate its privileges by directly attacking setuid, setgid, and
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fcap-using binaries; it will need to compromise something without the
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no_new_privs bit set first.
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In the future, other potentially dangerous kernel features could become
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available to unprivileged tasks if no_new_privs is set. In principle,
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several options to unshare(2) and clone(2) would be safe when
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no_new_privs is set, and no_new_privs + chroot is considerable less
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dangerous than chroot by itself.
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