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(p200300cbc72693001711035665507502.dip0.t-ipconnect.de. [2003:cb:c726:9300:1711:356:6550:7502]) by smtp.gmail.com with ESMTPSA id c21-20020a7bc855000000b003f17300c7dcsm24667685wml.48.2023.04.28.09.13.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Apr 2023 09:13:06 -0700 (PDT) Message-ID: <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com> Date: Fri, 28 Apr 2023 18:13:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default Content-Language: en-US To: "Kirill A . Shutemov" Cc: Lorenzo Stoakes , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , John Hubbard , Jan Kara , Pavel Begunkov , Mika Penttila , David Howells , Christoph Hellwig References: <6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@gmail.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 Organization: Red Hat In-Reply-To: <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org 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