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

[Jeff Xu <jeffxu@google.com>]

On Wed, May 22, 2024 at 7:25 PM Barnabás Pőcze <pobrn@protonmail.com> wrote:
>
> Hi
>
>
> 2024. május 23., csütörtök 1:23 keltezéssel, Andrew Morton <akpm@linux-foundation.org> írta:
>
> > On Wed, 15 May 2024 23:11:12 -0700 Jeff Xu <jeffxu@google.com> wrote:
> >
> > > 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/
> > >
> > > ...
> > >
> > > Reviewed-by: Jeff Xu <jeffxu@google.com>
> >
> > It's a change to a userspace API, yes?  Please let's have a detailed
> > description of why this is OK.  Why it won't affect any existing users.
>
> Yes, it is a uAPI change. To trigger user visible change, a program has to
>
>  - create a memfd
>    - with MFD_NOEXEC_SEAL,
>    - without MFD_ALLOW_SEALING;
>  - try to add seals / check the seals.
>
> This change in essence reverts the kernel's behaviour to that of Linux <6.3, where
> only `MFD_ALLOW_SEALING` enabled sealing. If a program works correctly on those
> kernels, it will likely work correctly after this change.
>
I agree with this.

The current memfd_test.c doesn't have good coverage sealable vs not_seable,
most tests are created with MFD_ALLOW_SEALING
I think the test_sysctl_set_sysctl0/1/2 need to add  cases for
no-sealable memfd.
because the change will also change the behavior of  the sysctl.
Do you want to add them as part of the patch ?


> I have looked through Debian Code Search and GitHub, searching for `MFD_NOEXEC_SEAL`.
> And I could find only a single breakage that this change would case: dbus-broker
> has its own memfd_create() wrapper that is aware of this implicit `MFD_ALLOW_SEALING`
> behaviour[0], and tries to work around it. This workaround will break. Luckily,
> however, as far as I could tell this only affects the test suite of dbus-broker,
> not its normal operations, so I believe it should be fine. I have prepared a PR
> with a fix[1].
>
Thanks for the investigation.

>
> >
> > Also, please let's give consideration to a -stable backport so that all
> > kernel versions will eventually behave in the same manner.
> >
> >
>
> I think that is a good idea, should I resend this with the `Cc: stable@...` tag or
> what should I do?
>
>
> Regards,
> Barnabás Pőcze
>
>
> [0]: https://github.com/bus1/dbus-broker/blob/9eb0b7e5826fc76cad7b025bc46f267d4a8784cb/src/util/misc.c#L114
> [1]: https://github.com/bus1/dbus-broker/pull/366