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(p200300cbc72693001711035665507502.dip0.t-ipconnect.de. [2003:cb:c726:9300:1711:356:6550:7502]) by smtp.gmail.com with ESMTPSA id p1-20020a05600c204100b003ef64affec7sm24665739wmg.22.2023.04.28.08.08.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Apr 2023 08:08:29 -0700 (PDT) Message-ID: <094d2074-5b69-5d61-07f7-9f962014fa68@redhat.com> Date: Fri, 28 Apr 2023 17:08:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Content-Language: en-US To: Jason Gunthorpe Cc: Lorenzo Stoakes , 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 , "Kirill A . Shutemov" , Pavel Begunkov , Mika Penttila , David Howells , Christoph Hellwig References: <6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@gmail.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default In-Reply-To: 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 16:35, Jason Gunthorpe wrote: > On Fri, Apr 28, 2023 at 04:20:46PM +0200, David Hildenbrand wrote: >> Sorry for jumping in late, I'm on vacation :) >> >> On 28.04.23 01:42, Lorenzo Stoakes wrote: >>> Writing to file-backed mappings which require folio dirty tracking using >>> GUP is a fundamentally broken operation, as kernel write access to GUP >>> mappings do not adhere to the semantics expected by a file system. >>> >>> A GUP caller uses the direct mapping to access the folio, which does not >>> cause write notify to trigger, nor does it enforce that the caller marks >>> the folio dirty. >> >> How should we enforce it? It would be a BUG in the GUP user. > > I hope we don't have these kinds of mistakes.. hard to enforce by > code. > I briefly played with the idea of only allowing to write-pin fs pages that are dirty (or the pte is dirty). If we adjust writeback code to leave such (maybe pinned) pages dirty, there would be no need to reset the pages dirty I guess. Just an idea, getting the writebackcode fixed (and race-free with GUP-fast) is the harder problem. >> This change has the potential to break existing setups. Simple example: >> libvirt domains configured for file-backed VM memory that also has a vfio >> device configured. It can easily be configured by users (evolving VM >> configuration, copy-paste etc.). And it works from a VM perspective, because >> the guest memory is essentially stale once the VM is shutdown and the pages >> were unpinned. At least we're not concerned about stale data on >> disk. > > I think this is broken today and we should block it. We know from > experiments with RDMA that doing exactly this triggers kernel oop's. > I never saw similar reports in the wild (especially targeted at RHEL), so is this still a current issue that has not been mitigated? Or is it just so hard to actually trigger? > Run your qemu config once, all the pages in the file become dirty. > > Run your qmeu config again and now all the dirty pages are longterm > pinned. > > Something eventually does writeback and FS cleans the page. At least vmscan does not touch pages that have additional references -- pageout() quits early. So that other thing doesn't happen that often I guess (manual fsync doesn't usually happen on VM memory if I am not wrong ...). > > Now close your VM and the page is dirtied without make write. FS is > inconsistent with the MM, kernel will eventually oops. > > I'm skeptical that anyone can actually do this combination of things > successfully without getting kernel crashes or file data corruption - > ie there is no real user to break. I am pretty sure that there are such VM users, because on the libvirt level it's completely unclear which features trigger what behavior :/ I remember (but did not check) that VM memory might usually get deleted whenever we usually shutdown a VM, another reason why we wouldn't see this in the wild. libvirt has the "discard" option exactly for that purpose, to be used with file-based guest memory. [1] Users tend to copy-paste domain XMLs + edit because it's just so hard to get right. Changing the VM backing to be backed from a file can be done with a one-line change in the libvirt XML. > >> With your changes, such VMs would no longer start, breaking existing user >> setups with a kernel update. > > Yes, as a matter of security we should break it. > > Earlier I suggested making this contingent on kernel lockdown >= > integrity, if actual users come and complain we should go to that > option. > >> Sure, we could warn, or convert individual users using a flag (io_uring). >> But maybe we should invest more energy on a fix? > > It has been years now, I think we need to admit a fix is still years > away. Blocking the security problem may even motivate more people to > work on a fix. Maybe we should make this a topic this year at LSF/MM (again?). At least we learned a lot about GUP, what might work, what might not work, and got a depper understanding (+ motivation to fix? :) ) the issue at hand. > > 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. Everything else sounds like band-aids to me, is insufficient, and might cause more harm than actually help IMHO. Especially the gup-fast case is extremely easy to work-around in malicious user space. [1] https://listman.redhat.com/archives/libvir-list/2018-May/msg00885.html -- Thanks, David / dhildenb