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Message-ID: <ZEvsx998gDFig/zq@x1n>
Date: Fri, 28 Apr 2023 11:56:55 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lstoakes@...il.com>,
Jason Gunthorpe <jgg@...dia.com>, linux-mm@...ck.org,
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
by default
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
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