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Date:   Fri, 28 Apr 2023 18:13:03 +0200
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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To:     "Kirill A . Shutemov" <kirill@shutemov.name>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>,
        Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
        Christian Benvenuti <benve@cisco.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Bernard Metzler <bmt@zurich.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Richard Cochran <richardcochran@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Mika Penttila <mpenttil@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@lst.de>
References: <6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@gmail.com>
 <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>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name>
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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.

-- 
Thanks,

David / dhildenb