Message-ID: <20230428165623.pqchgi5gtfhxd5b5@box.shutemov.name>
Date:   Fri, 28 Apr 2023 19:56:23 +0300
From:   "Kirill A . Shutemov" <kirill@...temov.name>
To:     David Hildenbrand <david@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, Lorenzo Stoakes <lstoakes@...il.com>,
        Jason Gunthorpe <jgg@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
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        Arnaldo Carvalho de Melo <acme@...nel.org>,
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        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Pavel Begunkov <asml.silence@...il.com>,
        Mika Penttila <mpenttil@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Fri, Apr 28, 2023 at 06:51:46PM +0200, David Hildenbrand wrote:
> On 28.04.23 18:39, Peter Xu wrote:
> > 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?
> > 
> 
> So, after we grabbed the page and made sure the the PTE didn't change (IOW,
> the PTE was stable while we processed it), the page can get unmapped (but
> not freed, because we hold a reference) and the VMA can theoretically go
> away (and as far as I understand, nothing stops the file from getting
> deleted, truncated etc).
> 
> So we might be looking at folio->mapping and the VMA is no longer there.
> Maybe even the file is no longer there.

No. VMA cannot get away before PTEs are unmapped and TLB is flushed. And
TLB flushing is serialized against GUP_fast().

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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