Re: [RFC PATCH v1 1/2] mm/memfd: Add support for F_SEAL_FUTURE_EXEC to memfd

Jeff Xu
2025-01-06T18:26:39Z

+ Kees because this is related to W^X memfd and security.

On Fri, Jan 3, 2025 at 7:14 AM Jann Horn <jannh@google.com> wrote:
>
> On Fri, Dec 6, 2024 at 7:19 PM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> > On Thu, Dec 05, 2024 at 05:09:22PM -0800, Isaac J. Manjarres wrote:
> > > + if (is_exec_sealed(seals)) {
> >
> > Are we intentionally disallowing a MAP_PRIVATE memfd's mapping's execution?
> > I've not tested this scenario so don't know if we somehow disallow this in
> > another way but note on write checks we only care about shared mappings.
> >
> > I mean one could argue that a MAP_PRIVATE situation is the same as copying
> > the data into an anon buffer and doing what you want with it, here you
> > could argue the same...
> >
> > So probably we should only care about VM_SHARED?
>
> FWIW I think it doesn't make sense to distinguish between
> shared/private mappings here - in the scenario described in the cover
> letter, it wouldn't matter that much to an attacker whether the
> mapping is shared or private (as long as the VMA contents haven't been
> CoWed already).
+1 on this.
The concept of blocking this for only shared mapping is questionable.

> But you're also right that in the scenario described,
> an attacker might also be able to create a writable+executable anon
> VMA and copy into that, or map another memfd that hasn't been sealed,
> or stuff like that. We can block such things - but not by only
> providing sealing operations on individual memfds. I think this
> instead requires policy that applies at the process level, either
> using system-wide SELinux policy or using process sandboxing APIs.
>