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[86.156.84.164]) by smtp.gmail.com with ESMTPSA id e18-20020adffc52000000b002efacde3fc7sm21459866wrs.35.2023.04.28.10.01.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Apr 2023 10:01:14 -0700 (PDT) Date: Fri, 28 Apr 2023 18:01:13 +0100 From: Lorenzo Stoakes To: David Hildenbrand Cc: Peter Xu , "Kirill A . Shutemov" , 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: <40fc128f-1978-42db-b9c1-77ac3c2cebfe@lucifer.local> References: <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> <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org 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. > This shouldn't be an issue though right? Because after a pup call unlocks the mmap_lock we're in the same situation anyway. GUP doesn't generally guarantee the mapping remains valid, only pinning the underlying folio. I'm thinking of respinning with a gup_fast component then, if a_ops is sufficient to identify file systems. We'll just revert to slow path for non-FOLL_FAST_ONLY cases. This would at least cover both FOLL_LONGTERM angles and could provoke some further interesting discussion :) > -- > Thanks, > > David / dhildenb >