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Date:   Fri, 28 Apr 2023 12:39:51 -0400
From:   Peter Xu <peterx@redhat.com>
To:     "Kirill A . Shutemov" <kirill@shutemov.name>
Cc:     David Hildenbrand <david@redhat.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
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        Oleg Nesterov <oleg@redhat.com>,
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
Message-ID: <ZEv2196tk5yWvgW5@x1n>
References: <afcc124e-7a9b-879c-dfdf-200426b84e24@redhat.com>
 <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>
 <6ddc7ac4-4091-632a-7b2c-df2005438ec4@redhat.com>
 <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name>
 <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com>
 <20230428162207.o3ejmcz7rzezpt6n@box.shutemov.name>
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On Fri, Apr 28, 2023 at 07:22:07PM +0300, Kirill A . Shutemov wrote:
> On Fri, Apr 28, 2023 at 06:13:03PM +0200, David Hildenbrand wrote:
> > On 28.04.23 18:09, Kirill A . Shutemov wrote:
> > > On Fri, Apr 28, 2023 at 05:43:52PM +0200, David Hildenbrand wrote:
> > > > On 28.04.23 17:34, 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().
> > > > 
> > > > IIRC, PageHuge() can also be called from GUP-fast and could special-case
> > > > hugetlb eventually, as it's table while we hold a (temporary) reference.
> > > > Shmem might be not so easy ...
> > > 
> > > page->mapping->a_ops should be enough to whitelist whatever fs you want.
> > > 
> > 
> > The issue is how to stabilize that from GUP-fast, such that we can safely
> > dereference the mapping. Any idea?
> > 
> > At least for anon page I know that page->mapping only gets cleared when
> > freeing the page, and we don't dereference the mapping but only check a
> > single flag stored alongside the mapping. Therefore, PageAnon() is fine in
> > GUP-fast context.
> 
> What codepath you are worry about that clears ->mapping on pages with
> non-zero refcount?
> 
> I can only think of truncate (and punch hole). READ_ONCE(page->mapping)
> and fail GUP_fast if it is NULL should be fine, no?
> 
> I guess we should consider if the inode can be freed from under us and the
> mapping pointer becomes dangling. But I think we should be fine here too:
> VMA pins inode and VMA cannot go away from under GUP.

Can vma still go away if during a fast-gup?

> 
> Hm?
> 
> (I didn't look close at GUP for a while and my reasoning might be off.)

Thanks,

-- 
Peter Xu