Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes]

On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote:
> > >
> > > Security is the primary case where we have historically closed uAPI
> > > items.
> >
> > As this patch
> >
> > 1) Does not tackle GUP-fast
> > 2) Does not take care of !FOLL_LONGTERM
> >
> > I am not convinced by the security argument in regard to this patch.
> >
> >
> > If we want to sells this as a security thing, we have to block it
> > *completely* and then CC stable.
>
> Regarding GUP-fast, to fix the issue there as well, I guess we could do
> something similar as I did in gup_must_unshare():
>
> If we're in GUP-fast (no VMA), and want to pin a !anon page writable,
> fallback to ordinary GUP. IOW, if we don't know, better be safe.

How do we determine it's non-anon in the first place? The check is on the
VMA. We could do it by following page tables down to folio and checking
folio->mapping for PAGE_MAPPING_ANON I suppose?

>
> Of course, this would prevent hugetlb/shmem from getting pinned writable
> during gup-fast. Unless we're able to whitelist them somehow in there.

We could degrade those to non-fast assuming not FOLL_FAST_ONLY. But it'd be
a pity.

>
>
> For FOLL_LONGTERM it might fairly uncontroversial. For everything else I'm
> not sure if there could be undesired side-effects.

Yeah this is why I pared the patch down to this alone :) there are
definitely concerns and issues with other cases, notably ptrace + friends
but obviously not only.

FOLL_LONGTERM is just the most egregious case.

>
> --
> Thanks,
>
> David / dhildenb
>