Hi
Gentle ping. Is there any chance we could move forward with this? I am not aware
of any breakage it would cause; but longer the wait, the higher the likelihood.
Regards,
Barnabás Pőcze
2024. június 30., vasárnap 20:49 keltezéssel, Barnabás Pőcze
<po...@protonmail.com> írta:
> `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-jef...@google.com/
> [1]: https://lore.kernel.org/lkml/20221202013404.163143-3-jef...@google.com/
> [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-da...@readahead.eu/
>
> Cc: sta...@vger.kernel.org
> Signed-off-by: Barnabás Pőcze <po...@protonmail.com>
> ---
>
> * v3:
> https://lore.kernel.org/linux-mm/20240611231409.3899809-1-jef...@chromium.org/
> * v2:
> https://lore.kernel.org/linux-mm/20240524033933.135049-1-jef...@google.com/
> * v1:
> https://lore.kernel.org/linux-mm/20240513191544.94754-1-po...@protonmail.com/
>
> 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
>
