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

[Barnabás Pőcze]

2024. május 23., csütörtök 22:44 keltezéssel, Jeff Xu <jeffxu@xxxxxxxxxx> írta:

> Hi Barnabás
> 
> Is that OK that I work on V2 ? It will be based on your V1 change and
> I will also add more test cases.

Sure, please go ahead. At the very end of this letter you'll find
the commit message that I would have sent in v2, maybe you can salvage
some of it.


Regards,
Barnabás Pőcze


> 
> Thanks
> -Jeff
> 
> -
> 
> On Thu, May 23, 2024 at 12:45 PM Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Wed, 22 May 2024 19:32:35 -0700 Jeff Xu <jeffxu@xxxxxxxxxx> wrote:
> >
> > > >
> > > > 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.
> > > >
> > > Unfortunately, this is a breaking change that might break a
> > > application if they do below:
> > > memfd_create("", MFD_NOEXEC_SEAL)
> > > fcntl(fd, F_ADD_SEALS, F_SEAL_WRITE); <-- this will fail in new
> > > semantics, due to mfd not being sealable.
> > >
> > > However, I still think the new semantics is a better, the reason is
> > > due to  the sysctl: memfd_noexec_scope
> > > Currently, when the sysctl  is set to MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL
> > > kernel adds MFD_NOEXEC_SEAL to memfd_create, and the memfd  becomes sealable.
> > > E.g.
> > > When the sysctl is set to MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL
> > > The app calls memfd_create("",0)
> > > application will get sealable memfd, which might be a surprise to application.
> > >
> > > If the app doesn't want this behavior, they will need one of two below
> > > in current implementation.
> > > 1>
> > > set the sysctl: memfd_noexec_scope to 0.
> > > So the kernel doesn't overwrite the mdmfd_create
> > >
> > > 2>
> > > modify their code  to get non-sealable NOEXEC memfd.
> > > memfd_create("", MEMFD_NOEXEC_SCOPE_NOEXEC)
> > > fcntl(fd, F_ADD_SEALS, F_SEAL_SEAL)
> > >
> > > The new semantics works better with the sysctl.
> > >
> > > Since memfd noexec is new, maybe there is no application using the
> > > MFD_NOEXEC_SEAL to create
> > > sealable memfd. They mostly likely use
> > > memfd(MFD_NOEXEC_SEAL|MFD_ALLOW_SEALING) instead.
> > > I think it might benefit in the long term with the new semantics.
> >
> > Yes, it's new so I expect any damage will be small.  Please prepare a
> > v2 which fully explains/justifies the thinking for this
> > non-backward-compatible change and which include the cc:stable.
> >
> >
> 

---

memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW_SEALING`

`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 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].

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@xxxxxxxxxx/
[1]: https://lore.kernel.org/lkml/20221202013404.163143-3-jeffxu@xxxxxxxxxx/
[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@xxxxxxxxxxxx/

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 105ff5339f498a ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
Signed-off-by: Barnabás Pőcze <pobrn@xxxxxxxxxxxxxx>
Reviewed-by: Jeff Xu <jeffxu@xxxxxxxxxx>
Reviewed-by: David Rheinsberg <david@xxxxxxxxxxxx>

---