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Sun, 23 Apr 2023 12:40:57 -0700 (PDT) Date: Sun, 23 Apr 2023 20:40:53 +0100 From: Lorenzo Stoakes To: Simon Horman Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , 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 Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7082D160007 X-Stat-Signature: zyob1aafwzabcs8k1oecwzjo5szssirq X-HE-Tag: 1682278860-24692 X-HE-Meta: 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 RVe+bOWl MY+ont3KxZH5KwXRIvkM4lauM4AJqR4xYj60UXw9t8xV3FsUglRyUOHE9kx4teOHkGKDYZZclipk2oFMj88GJ7WFrSGWGmOjBF5LzcoqusI7MbZx81JiXGTt7G4VJOppRLgVdDMhKclu6UsLqFFi7f8K00pxy3vDcPKhvVfygC6l0ZHpN0VLmWFEhcbmM1bwk2gjGpWvBs7v214znw8sXvPWXGr9B/4cjAqCR0Wy35Pzk07mLsqraxKz7kq7EirkdzKO61ugNn7+xWAKCu22oJfYTRc3oEan8b3bEKiAswbqH+7jOyVuRHY/5MgMmJ9WvgbZJQpVeQPHDD0PXZEgdaCNJ/CDsBdvoZTpUJJXjE1Uu8l4Q4uwBYiRuM/L+bJeBkUVYIY4pXo1ld1BknCOunZufqojRvgoTV3zNBo/BRzvraKAH7KZFRG0zWdSo1LRwEe9qfmFv4S9MBfBu2al5SDRI4Q== 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 Sun, Apr 23, 2023 at 09:32:01PM +0200, Simon Horman wrote: > On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote: > > It isn't safe to write to file-backed mappings as GUP does not ensure that > > the semantics associated with such a write are performed correctly, for > > instance filesystems which rely upon write-notify will not be correctly > > notified. > > > > There are exceptions to this - shmem and hugetlb mappings are (in effect) > > anonymous mappings by other names so we do permit this operation in these > > cases. > > > > In addition, if no pinning takes place (neither FOLL_GET nor FOLL_PIN is > > specified and neither flags gets implicitly set) then no writing can occur > > so we do not perform the check in this instance. > > > > This is an important exception, as populate_vma_page_range() invokes > > __get_user_pages() in this way (and thus so does __mm_populate(), used by > > MAP_POPULATE mmap() and mlock() invocations). > > > > There are GUP users within the kernel that do nevertheless rely upon this > > behaviour, so we introduce the FOLL_ALLOW_BROKEN_FILE_MAPPING flag to > > explicitly permit this kind of GUP access. > > > > This is required in order to not break userspace in instances where the > > uAPI might permit file-mapped addresses - a number of RDMA users require > > this for instance, as do the process_vm_[read/write]v() system calls, > > /proc/$pid/mem, ptrace and SDT uprobes. Each of these callers have been > > updated to use this flag. > > > > Making this change is an important step towards a more reliable GUP, and > > explicitly indicates which callers might encouter issues moving forward. > > nit: s/encouter/encounter/ > Ack, I always seem to leave at least one or two easter egg spelling mistakes in :) Will fix up on next respin (in unlikely event of no further review, hopefully Andrew would pick up!)