[PATCH v2 0/2] memfd: fix MFD_NOEXEC_SEAL to be non-sealable

[jeffxu@chromium.org]

From: Jeff Xu <jeffxu@google.com>

By default, memfd_create() creates a non-sealable MFD, unless the
MFD_ALLOW_SEALING flag is set.

When the MFD_NOEXEC_SEAL flag is initially introduced, the MFD created
with that flag is sealable, even though MFD_ALLOW_SEALING is not set.
This patch changes MFD_NOEXEC_SEAL to be non-sealable by default,
unless MFD_ALLOW_SEALING is explicitly set.

This is a non-backward compatible change. However, as MFD_NOEXEC_SEAL
is new, we expect not many applications will rely on the nature of
MFD_NOEXEC_SEAL being sealable. In most cases, the application already
sets MFD_ALLOW_SEALING if they need a sealable MFD.

Additionally, this enhances the useability of  pid namespace sysctl
vm.memfd_noexec. When vm.memfd_noexec equals 1 or 2, the kernel will
add MFD_NOEXEC_SEAL if mfd_create does not specify MFD_EXEC or
MFD_NOEXEC_SEAL, and the addition of MFD_NOEXEC_SEAL enables the MFD
to be sealable. This means, any application that does not desire this
behavior will be unable to utilize vm.memfd_noexec = 1 or 2 to
migrate/enforce non-executable MFD. This adjustment ensures that
applications can anticipate that the sealable characteristic will
remain unmodified by vm.memfd_noexec.

This patch was initially developed by Barnabás Pőcze, and Barnabás
used Debian Code Search and GitHub to try to find potential breakages
and could only find a single one. Dbus-broker's memfd_create() wrapper
is aware of this implicit `MFD_ALLOW_SEALING` behavior, and tries to
work around it [1]. This workaround will break. Luckily, this only
affects the test suite, it does not affect
the normal operations of dbus-broker. There is a PR with a fix[2]. In
addition, David Rheinsberg also raised similar fix in [3]

[1]: https://github.com/bus1/dbus-broker/blob/9eb0b7e5826fc76cad7b025bc46f267d4a8784cb/src/util/misc.c#L114
[2]: https://github.com/bus1/dbus-broker/pull/366
[3]: https://lore.kernel.org/lkml/20230714114753.170814-1-david@readahead.eu/


History
======
V2:
  update commit message.
  add testcase for vm.memfd_noexec
  add documentation.

V1:
https://lore.kernel.org/lkml/20240513191544.94754-1-pobrn@protonmail.com/


Jeff Xu (2):
  memfd: fix MFD_NOEXEC_SEAL to be non-sealable by default
  memfd:add MEMFD_NOEXEC_SEAL documentation

 Documentation/userspace-api/index.rst      |  1 +
 Documentation/userspace-api/mfd_noexec.rst | 90 ++++++++++++++++++++++
 mm/memfd.c                                 |  9 +--
 tools/testing/selftests/memfd/memfd_test.c | 26 ++++++-
 4 files changed, 120 insertions(+), 6 deletions(-)
 create mode 100644 Documentation/userspace-api/mfd_noexec.rst

-- 
2.45.1.288.g0e0cd299f1-goog