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

[Peter Xu]

On Fri, Apr 28, 2023 at 05:34:35PM +0200, David Hildenbrand wrote:
> 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().

Hmm.. Is it a good idea at all to sacrifise all "!anon" fast-gups for this?
People will silently got degrade even on legal pins on shmem/hugetlb, I
think, which seems to be still a very major use case.

-- 
Peter Xu