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doc: ReSTify no_new_privs.txt

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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>
This commit is contained in:
Kees Cook 2017-05-13 04:51:38 -07:00 committed by Jonathan Corbet
parent c061f33f35
commit 40fde647cc
2 changed files with 26 additions and 19 deletions
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1
Documentation/userspace-api/index.rst
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@ -16,6 +16,7 @@ place where this information is gathered.
.. toctree::
:maxdepth: 2
no_new_privs
seccomp_filter
unshare

44
Documentation/prctl/no_new_privs.txt → Documentation/userspace-api/no_new_privs.rst
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@ -1,3 +1,7 @@
======================
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
@ -5,53 +9,55 @@ 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
- 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
``/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
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
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).
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
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.
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:
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.
if ``no_new_privs`` is set.
- By itself, no_new_privs can be used to reduce the attack surface
- 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
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.
``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
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.