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Fri, 28 Apr 2023 12:22:08 -0400 (EDT) Received: by box.shutemov.name (Postfix, from userid 1000) id 28C2E1041AE; Fri, 28 Apr 2023 19:22:07 +0300 (+03) Date: Fri, 28 Apr 2023 19:22:07 +0300 From: "Kirill A . Shutemov" To: David Hildenbrand 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 Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default Message-ID: <20230428162207.o3ejmcz7rzezpt6n@box.shutemov.name> 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> <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com> X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: D335E1C001B X-Stat-Signature: bdorpr34j3hmeskyw8hxin8fo4kzcmza X-HE-Tag: 1682698937-694015 X-HE-Meta: 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 kCMl91+O crQOxcUbPtQ31aIjiWY+nGRy+p19Gpzd6gj6xqyF5xR7bN/sF2V0q1KkStijgjc+QEv2JJUtUSSoTzeZMj/znr7nYJSrBO2nIVXeUdDP13eJBpIkD0nn5o//ItMJqdw4R5jjShD8I1FH+nd+95kyYm28r28d15tXX+vDJ53weS5ervb9YhzxZWqnlhwS54RnhngEX7FoNu/wbhOHWzpsQvtlP7w42IZ/yPYzip7HR9QkoCPDXEIPe24sClmVL5I+7K9YEH//XGsfKCGqskRtTJFAoWNXGM9nVtteKcVg6eQxAZb4qu7Btk30pHhD5sJVOEgiE97T3Q0dIL1DPeSIop84H6ZxTMR3fT9wJL+bGTJquCYgCDBuuTnhjIQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. Hm? (I didn't look close at GUP for a while and my reasoning might be off.) -- Kiryl Shutsemau / Kirill A. Shutemov