64 lines
2.9 KiB
ReStructuredText
64 lines
2.9 KiB
ReStructuredText
======================
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No New Privileges Flag
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======================
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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::
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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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