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(p200300cbc72693001711035665507502.dip0.t-ipconnect.de. [2003:cb:c726:9300:1711:356:6550:7502]) by smtp.gmail.com with ESMTPSA id c7-20020a5d4cc7000000b002fa5a73bf9bsm21531981wrt.89.2023.04.28.10.05.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Apr 2023 10:05:40 -0700 (PDT) Message-ID: <3d7fcfab-e445-1dc7-f000-9fbe7bea04c0@redhat.com> Date: Fri, 28 Apr 2023 19:05:38 +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: Lorenzo Stoakes 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 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> <40fc128f-1978-42db-b9c1-77ac3c2cebfe@lucifer.local> From: David Hildenbrand Organization: Red Hat In-Reply-To: <40fc128f-1978-42db-b9c1-77ac3c2cebfe@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On 28.04.23 19:01, Lorenzo Stoakes wrote: > 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. Yes. But the issue here is rather dereferencing something that has already been freed, eventually leading to undefined behavior. Maybe de-referencing folio->mapping is fine ... but yes, we could handle that optimization in a separate patch. -- Thanks, David / dhildenb