[PATCH v6 0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC

[jeffxu]

From: Jeff Xu <jeffxu@xxxxxxxxxx>

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 is V6 version of patch: see [4] [5] [6] [7] 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@xxxxxxxxxx/
[6] https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@xxxxxxxxxx/
[7] https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@xxxxxxxxxx/

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
  mm/memfd: Add write seals when apply SEAL_EXEC to executable memfd
  selftests/memfd: add tests for MFD_NOEXEC_SEAL MFD_EXEC
  mm/memfd: security hook for memfd_create

 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                     |   5 +
 kernel/pid_sysctl.h                        |  59 ++++
 mm/memfd.c                                 |  61 +++-
 mm/shmem.c                                 |   6 +
 security/security.c                        |  13 +
 tools/testing/selftests/memfd/fuse_test.c  |   1 +
 tools/testing/selftests/memfd/memfd_test.c | 348 ++++++++++++++++++++-
 13 files changed, 525 insertions(+), 3 deletions(-)
 create mode 100644 kernel/pid_sysctl.h


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