Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW_SEALING`

[Jeff Xu <jeffxu@google.com>]

On Mon, May 13, 2024 at 12:15 PM Barnabás Pőcze <pobrn@protonmail.com> wrote:
>
> `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, currently, it also 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 suprising for application developers,
> there is no documentation that would reveal that `MFD_NOEXEC_SEAL`
> has the additional effect of `MFD_ALLOW_SEALING`.
>
Ya, I agree that there should be documentation, such as a man page. I will
work on that.

> So do not remove `F_SEAL_SEAL` when `MFD_NOEXEC_SEAL` is requested.
> This is technically an ABI break, but it seems very unlikely that an
> application would depend on this behaviour (unless by accident).
>
> [0]: https://lore.kernel.org/lkml/20220805222126.142525-3-jeffxu@google.com/
> [1]: https://lore.kernel.org/lkml/20221202013404.163143-3-jeffxu@google.com/
>
> Fixes: 105ff5339f498a ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
> Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
> ---
>
> Or did I miss the explanation as to why MFD_NOEXEC_SEAL should
> imply MFD_ALLOW_SEALING? If so, please direct me to it and
> sorry for the noise.
>
Previously I might be thinking  MFD_NOEXEC_SEAL implies
MFD_ALLOW_SEALING because MFD_NOEXEC_SEAL seals F_SEAL_EXEC, and
sealing is added only when MFD_ALLOW_SEALING is set.

I agree your patch handles this better, e.g.
mfd_create(MFD_NOEXEC_SEAL) will have F_SEAL_SEAL and F_SEAL_EXEC
mfd_create(MFD_NOEXEC_SEAL|MFD_ALLOW_SEALING) will have F_SEAL_EXEC


> ---
>  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 18f585684e20..b6a7ad68c3c1 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);
>  }
> --
> 2.45.0
>

Reviewed-by: Jeff Xu <jeffxu@google.com>

Thanks!
-Jeff