linux-kernel - Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW

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Date: Thu, 23 May 2024 02:25:04 +0000
From: Barnabás Pőcze <pobrn@...tonmail.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jeff Xu <jeffxu@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, dmitry.torokhov@...il.com, dverkamp@...omium.org, hughd@...gle.com, jorgelo@...omium.org, skhan@...uxfoundation.org, keescook@...omium.org
Subject: Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW_SEALING`

Hi


2024. május 23., csütörtök 1:23 keltezéssel, Andrew Morton <akpm@...ux-foundation.org> írta:

> On Wed, 15 May 2024 23:11:12 -0700 Jeff Xu <jeffxu@...gle.com> wrote:
> 
> > On Mon, May 13, 2024 at 12:15 PM Barnabás Pőcze <pobrn@...tonmail.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@...gle.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 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].


> 
> 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