linux-kernel - [PATCH v3] mm/memfd: MFD_NOEXEC_SEAL and MFD

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Date:   Fri,  2 Dec 2022 01:33:58 +0000
From:   jeffxu@...omium.org
To:     skhan@...uxfoundation.org, keescook@...omium.org
Cc:     akpm@...ux-foundation.org, dmitry.torokhov@...il.com,
        dverkamp@...omium.org, hughd@...gle.com, jeffxu@...gle.com,
        jorgelo@...omium.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        mnissler@...omium.org, jannh@...gle.com,
        linux-hardening@...r.kernel.org, Jeff Xu <jeffxu@...omium.org>
Subject: [PATCH v3] mm/memfd: MFD_NOEXEC_SEAL and MFD_EXEC

From: Jeff Xu <jeffxu@...omium.org>

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 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 is V3 version of patch: see [4] [5] for previous versions.

[1] https://crbug.com/1305411
[2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20memfd%20escalation&can=1
[3] https://lwn.net/Articles/781013/
[4] https://lwn.net/Articles/890096/
[5] https://lore.kernel.org/lkml/20220805222126.142525-1-jeffxu@google.com/

Daniel Verkamp (2):
  mm/memfd: add F_SEAL_EXEC
  selftests/memfd: add tests for F_SEAL_EXEC

Jeff Xu (4):
  mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC
  selftests/memfd: add tests for MFD_NOEXEC_SEAL MFD_EXEC
  mm/memfd: security hook for memfd_create
  mm/memfd: Add write seals when apply SEAL_EXEC to executable memfd

 include/linux/lsm_hook_defs.h              |   1 +
 include/linux/lsm_hooks.h                  |   4 +
 include/linux/pid_namespace.h              |  19 ++
 include/linux/security.h                   |   6 +
 include/uapi/linux/fcntl.h                 |   1 +
 include/uapi/linux/memfd.h                 |   4 +
 kernel/pid_namespace.c                     |  47 ++++
 mm/memfd.c                                 |  54 +++-
 mm/shmem.c                                 |   6 +
 tools/testing/selftests/memfd/fuse_test.c  |   1 +
 tools/testing/selftests/memfd/memfd_test.c | 305 ++++++++++++++++++++-
 11 files changed, 445 insertions(+), 3 deletions(-)

base-commit: eb7081409f94a9a8608593d0fb63a1aa3d6f95d8
-- 
2.39.0.rc0.267.gcb52ba06e7-goog