`MFD_NOEXEC_SEAL` should remove the executable bits and set `F_SEAL_EXEC`
to prevent further modifications to the executable bits as per the comment
in the uapi header file:
not executable and sealed to prevent changing to executable
However, commit 105ff5339f498a ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
that introduced this feature made it so that `MFD_NOEXEC_SEAL` unsets
`F_SEAL_SEAL`, essentially acting as a superset of `MFD_ALLOW_SEALING`.
Nothing implies that it should be so, and indeed up until the second version
of the of the patchset[0] that introduced `MFD_EXEC` and `MFD_NOEXEC_SEAL`,
`F_SEAL_SEAL` was not removed, however, it was changed in the third revision
of the patchset[1] without a clear explanation.
This behaviour is surprising for application developers, there is no
documentation that would reveal that `MFD_NOEXEC_SEAL` has the additional
effect of `MFD_ALLOW_SEALING`. Additionally, combined with `vm.memfd_noexec=2`
it has the effect of making all memfds initially sealable.
So do not remove `F_SEAL_SEAL` when `MFD_NOEXEC_SEAL` is requested,
thereby returning to the pre-Linux 6.3 behaviour of only allowing
sealing when `MFD_ALLOW_SEALING` is specified.
Now, this is technically a uapi break. However, the damage is expected
to be minimal. To trigger user visible change, a program has to do the
following steps:
- create memfd:
- with `MFD_NOEXEC_SEAL`,
- without `MFD_ALLOW_SEALING`;
- try to add seals / check the seals.
But that seems unlikely to happen intentionally since this change
essentially reverts the kernel's behaviour to that of Linux <6.3,
so if a program worked correctly on those older kernels, it will
likely work correctly after this change.
I have used Debian Code Search and GitHub to try to find potential
breakages, and I could only find a single one. dbus-broker's
memfd_create() wrapper is aware of this implicit `MFD_ALLOW_SEALING`
behaviour, and tries to work around it[2]. 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[3].
I also carried out a smoke test by building a kernel with this change
and booting an Arch Linux system into GNOME and Plasma sessions.
There was also a previous attempt to address this peculiarity by
introducing a new flag[4].
[0]: https://lore.kernel.org/lkml/20220805222126.142525-3-jeffxu@xxxxxxxxxx/
[1]: https://lore.kernel.org/lkml/20221202013404.163143-3-jeffxu@xxxxxxxxxx/
[2]: https://github.com/bus1/dbus-broker/blob/9eb0b7e5826fc76cad7b025bc46f267d4a8784cb/src/util/misc.c#L114
[3]: https://github.com/bus1/dbus-broker/pull/366
[4]: https://lore.kernel.org/lkml/20230714114753.170814-1-david@xxxxxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Barnabás Pőcze <pobrn@xxxxxxxxxxxxxx>
---
* v3: https://lore.kernel.org/linux-mm/20240611231409.3899809-1-jeffxu@xxxxxxxxxxxx/
* v2: https://lore.kernel.org/linux-mm/20240524033933.135049-1-jeffxu@xxxxxxxxxx/
* v1: https://lore.kernel.org/linux-mm/20240513191544.94754-1-pobrn@xxxxxxxxxxxxxx/
This fourth version returns to removing the inconsistency as opposed to documenting
its existence, with the same code change as v1 but with a somewhat extended commit
message. This is sent because I believe it is worth at least a try; it can be easily
reverted if bigger application breakages are discovered than initially imagined.
---
mm/memfd.c | 9 ++++-----
tools/testing/selftests/memfd/memfd_test.c | 2 +-
2 files changed, 5 insertions(+), 6 deletions(-)
diff --git a/mm/memfd.c b/mm/memfd.c
index 7d8d3ab3fa37..8b7f6afee21d 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -356,12 +356,11 @@ SYSCALL_DEFINE2(memfd_create,
inode->i_mode &= ~0111;
file_seals = memfd_file_seals_ptr(file);
- if (file_seals) {
- *file_seals &= ~F_SEAL_SEAL;
+ if (file_seals)
*file_seals |= F_SEAL_EXEC;
- }
- } else if (flags & MFD_ALLOW_SEALING) {
- /* MFD_EXEC and MFD_ALLOW_SEALING are set */
+ }
+
+ if (flags & MFD_ALLOW_SEALING) {
file_seals = memfd_file_seals_ptr(file);
if (file_seals)
*file_seals &= ~F_SEAL_SEAL;
diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index 95af2d78fd31..7b78329f65b6 100644
--- a/tools/testing/selftests/memfd/memfd_test.c
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -1151,7 +1151,7 @@ static void test_noexec_seal(void)
mfd_def_size,
MFD_CLOEXEC | MFD_NOEXEC_SEAL);
mfd_assert_mode(fd, 0666);
- mfd_assert_has_seals(fd, F_SEAL_EXEC);
+ mfd_assert_has_seals(fd, F_SEAL_SEAL | F_SEAL_EXEC);
mfd_fail_chmod(fd, 0777);
close(fd);
}
--
2.45.2
