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

[David Hildenbrand]

On 28.04.23 17:33, Lorenzo Stoakes wrote:
> 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?

PageAnon(page) can be called from GUP-fast after grabbing a reference. 
See gup_must_unshare().

> > 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.

--
Thanks,

David / dhildenb