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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Jan 06, 2025 at 04:44:33PM -0800, Kees Cook wrote:
> On Mon, Jan 06, 2025 at 10:26:27AM -0800, Jeff Xu wrote:
> > + 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.
>
> Right -- why does sharedness matter? It seems more robust to me to not
> create a corner case but rather apply the flag/behavior universally?
>

I'm struggling to understand what you are protecting against, if I can receive a
buffer '-not executable-'. But then copy it into another buffer I mapped, and
execute it?

I mean am I missing something? It's very possible :)

The cost is complexity. And the difference between mappings which are shared and
those which are private and moreso MAP_PRIVATE of an fd are actually quite a lot
internally (go look at anon_vma code if you have the great benefit of not yet
doing so to see the deepest, darkest, 9th circle of complexity hell :>).

Again, I may be missing the point here or misunderstanding the apparent attack
vector, but this is where I'm coming from.

> --
> Kees Cook