mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-16 01:46:51 +07:00
40fde647cc
This updates no_new_privs documentation to ReST markup and adds it to the user-space API documentation. Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
64 lines
2.9 KiB
ReStructuredText
64 lines
2.9 KiB
ReStructuredText
======================
|
|
No New Privileges Flag
|
|
======================
|
|
|
|
The execve system call can grant a newly-started program privileges that
|
|
its parent did not have. The most obvious examples are setuid/setgid
|
|
programs and file capabilities. To prevent the parent program from
|
|
gaining these privileges as well, the kernel and user code must be
|
|
careful to prevent the parent from doing anything that could subvert the
|
|
child. For example:
|
|
|
|
- The dynamic loader handles ``LD_*`` environment variables differently if
|
|
a program is setuid.
|
|
|
|
- chroot is disallowed to unprivileged processes, since it would allow
|
|
``/etc/passwd`` to be replaced from the point of view of a process that
|
|
inherited chroot.
|
|
|
|
- The exec code has special handling for ptrace.
|
|
|
|
These are all ad-hoc fixes. The ``no_new_privs`` bit (since Linux 3.5) is a
|
|
new, generic mechanism to make it safe for a process to modify its
|
|
execution environment in a manner that persists across execve. Any task
|
|
can set ``no_new_privs``. Once the bit is set, it is inherited across fork,
|
|
clone, and execve and cannot be unset. With ``no_new_privs`` set, ``execve()``
|
|
promises not to grant the privilege to do anything that could not have
|
|
been done without the execve call. For example, the setuid and setgid
|
|
bits will no longer change the uid or gid; file capabilities will not
|
|
add to the permitted set, and LSMs will not relax constraints after
|
|
execve.
|
|
|
|
To set ``no_new_privs``, use::
|
|
|
|
prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
|
|
|
|
Be careful, though: LSMs might also not tighten constraints on exec
|
|
in ``no_new_privs`` mode. (This means that setting up a general-purpose
|
|
service launcher to set ``no_new_privs`` before execing daemons may
|
|
interfere with LSM-based sandboxing.)
|
|
|
|
Note that ``no_new_privs`` does not prevent privilege changes that do not
|
|
involve ``execve()``. An appropriately privileged task can still call
|
|
``setuid(2)`` and receive SCM_RIGHTS datagrams.
|
|
|
|
There are two main use cases for ``no_new_privs`` so far:
|
|
|
|
- Filters installed for the seccomp mode 2 sandbox persist across
|
|
execve and can change the behavior of newly-executed programs.
|
|
Unprivileged users are therefore only allowed to install such filters
|
|
if ``no_new_privs`` is set.
|
|
|
|
- By itself, ``no_new_privs`` can be used to reduce the attack surface
|
|
available to an unprivileged user. If everything running with a
|
|
given uid has ``no_new_privs`` set, then that uid will be unable to
|
|
escalate its privileges by directly attacking setuid, setgid, and
|
|
fcap-using binaries; it will need to compromise something without the
|
|
``no_new_privs`` bit set first.
|
|
|
|
In the future, other potentially dangerous kernel features could become
|
|
available to unprivileged tasks if ``no_new_privs`` is set. In principle,
|
|
several options to ``unshare(2)`` and ``clone(2)`` would be safe when
|
|
``no_new_privs`` is set, and ``no_new_privs`` + ``chroot`` is considerable less
|
|
dangerous than chroot by itself.
|