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

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).

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