Hi

On Sun, Jun 30, 2024 at 11:49 AM 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, 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.

The documentation is in linux main (653c5c75115c), I hope this gives clarity to the usage of MFD_NOEXEC_SEAL flag to application developers, furthermore I'm working on man page for memfd_create.

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.

During V3 patch discussion, I sent my reasoning, but here are summaries:

- As one might have noticed, unlike other flags in memfd_create, MFD_NOEXEC_SEAL is actually a combination of multiple flags. The idea is to make it easier to use memfd in the most common way, which is NOEXEC + F_SEAL_EXEC + MFD_ALLOW_SEALING.

- The new sysctl vm.noexec = 1 helps existing applications move to a more secure way of using memfd. IMO, MFD_ALLOW_SEALING is included by default because most applications would rather have it than not. In any case, an app can set F_SEAL_SEAL to disable the sealing.

- MFD_NOEXEC_SEAL has been added for more than one year, multiple applications and distributions have backported and utilized it. Altering ABI now presents a degree of risk and may lead to disruptions.

Best regards, -Jeff

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].

Cc: stable@vger.kernel.org Signed-off-by: Barnabás Pőcze pobrn@protonmail.com

---

• v3: https://lore.kernel.org/linux-mm/20240611231409.3899809-1-jeffxu@chromium.or...
• v2: https://lore.kernel.org/linux-mm/20240524033933.135049-1-jeffxu@google.com/
• v1: https://lore.kernel.org/linux-mm/20240513191544.94754-1-pobrn@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