linux-kernel - [RFC PATCH 0/3] memfd: cleanups for vm.memfd

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Date:   Fri, 14 Jul 2023 00:33:45 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Jeff Xu <jeffxu@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Daniel Verkamp <dverkamp@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        YueHaibing <yuehaibing@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: [RFC PATCH 0/3] memfd: cleanups for vm.memfd_noexec

It seems that the most critical issue with vm.memfd_noexec=2 (the fact
that passing MFD_EXEC would bypass it entirely[1]) has been fixed in
Andrew's tree[2], but there are still some outstanding issues that need
to be addressed:

 * The dmesg warnings are pr_warn_once, which on most systems means that
   they will be used up by systemd or some other boot process and
   userspace developers will never see it. The original patch posted to
   the ML used pr_warn_ratelimited but the merged patch had it changed
   (with a comment about it being "per review"), but given that the
   current warnings are useless, pr_warn_ratelimited makes far more
   sense.

 * vm.memfd_noexec=2 shouldn't reject old-style memfd_create(2) syscalls
   because it will make it far to difficult to ever migrate. Instead it
   should imply MFD_EXEC.

 * The racheting mechanism for vm.memfd_noexec doesn't make sense as a
   security mechanism because a CAP_SYS_ADMIN capable user can create
   executable binaries in a hidden tmpfs very easily, not to mention the
   many other things they can do.

 * The memfd selftests would not exit with a non-zero error code when
   certain tests that ran in a forked process (specifically the ones
   related to MFD_EXEC and MFD_NOEXEC_SEAL) failed.

(This patchset is based on top of Jeff Xu's patches[2] fixing the
MFD_EXEC bug in vm.memfd_noexec=2.)

[1]: https://lore.kernel.org/all/ZJwcsU0vI-nzgOB_@codewreck.org/
[2]: https://lore.kernel.org/all/20230705063315.3680666-1-jeffxu@google.com/

Aleksa Sarai (3):
  memfd: cleanups for vm.memfd_noexec handling
  memfd: remove racheting feature from vm.memfd_noexec
  selftests: memfd: error out test process when child test fails

 include/linux/pid_namespace.h              | 16 +++------
 kernel/pid_sysctl.h                        |  7 ----
 mm/memfd.c                                 | 32 +++++++----------
 tools/testing/selftests/memfd/memfd_test.c | 41 ++++++++++++++++++----
 4 files changed, 51 insertions(+), 45 deletions(-)

-- 
2.41.0