mm/memfd: add F_SEAL_EXEC

Patch series "mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC", v8.

Since Linux introduced the memfd feature, memfd have always had their
execute bit set, and the memfd_create() syscall doesn't allow setting it
differently.

However, in a secure by default system, such as ChromeOS, (where all
executables should come from the rootfs, which is protected by Verified
boot), this executable nature of memfd opens a door for NoExec bypass and
enables “confused deputy attack”.  E.g, in VRP bug [1]: cros_vm
process created a memfd to share the content with an external process,
however the memfd is overwritten and used for executing arbitrary code and
root escalation.  [2] lists more VRP in this kind.

On the other hand, executable memfd has its legit use, runc uses memfd’s
seal and executable feature to copy the contents of the binary then
execute them, for such system, we need a solution to differentiate runc's
use of executable memfds and an attacker's [3].

To address those above, this set of patches add following:
1> Let memfd_create() set X bit at creation time.
2> Let memfd to be sealed for modifying X bit.
3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
   X bit.For example, if a container has vm.memfd_noexec=2, then
   memfd_create() without MFD_NOEXEC_SEAL will be rejected.
4> A new security hook in memfd_create(). This make it possible to a new
   LSM, which rejects or allows executable memfd based on its security policy.


This patch (of 5):

The new F_SEAL_EXEC flag will prevent modification of the exec bits:
written as traditional octal mask, 0111, or as named flags, S_IXUSR |
S_IXGRP | S_IXOTH.  Any chmod(2) or similar call that attempts to modify
any of these bits after the seal is applied will fail with errno EPERM.

This will preserve the execute bits as they are at the time of sealing, so
the memfd will become either permanently executable or permanently
un-executable.

Link: https://lkml.kernel.org/r/20221215001205.51969-1-jeffxu@google.com
Link: https://lkml.kernel.org/r/20221215001205.51969-2-jeffxu@google.com


Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Co-developed-by: Jeff Xu <jeffxu@google.com>
Signed-off-by: Jeff Xu <jeffxu@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>