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Date:   Fri, 28 Apr 2023 11:56:55 -0400
From:   Peter Xu <peterx@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>,
        Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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References: <6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@gmail.com>
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 <ZEvZtIb2EDb/WudP@nvidia.com>
 <094d2074-5b69-5d61-07f7-9f962014fa68@redhat.com>
 <400da248-a14e-46a4-420a-a3e075291085@redhat.com>
 <077c4b21-8806-455f-be98-d7052a584259@lucifer.local>
 <62ec50da-5f73-559c-c4b3-bde4eb215e08@redhat.com>
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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