Re: [PATCH v4] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW_SEALING`

["Barnabás Pőcze" <pobrn@protonmail.com>]

Hi


Gentle ping again. I am still hoping we can move forward with this.


Regards,
Barnabás Pőcze


2024. szeptember 28., szombat 0:09 keltezéssel, Barnabás Pőcze <pobrn@protonmail.com> írta:

> 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 <pobrn@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-jeffxu@google.com/
> > [1]: https://lore.kernel.org/lkml/20221202013404.163143-3-jeffxu@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-david@readahead.eu/
> >
> > 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.org/
> > * 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
> >