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Howlett" , "Vlastimil Babka" , "Jann Horn" , "Pedro Falcato" , "David Hildenbrand" , "Mike Rapoport" , "Suren Baghdasaryan" , "Michal Hocko" , "Jonathan Corbet" , "Greg Kroah-Hartman" , "Dennis Dalessandro" , "Jason Gunthorpe" , "Leon Romanovsky" , "Paul Moore" , "Stephen Smalley" , "Jaroslav Kysela" , "Takashi Iwai" , "Alexei Starovoitov" , "Daniel Borkmann" , "Andrii Nakryiko" , "Eduard Zingerman" , "Kumar Kartikeya Dwivedi" , "Baolin Wang" , "Nico Pache" , "Ryan Roberts" , "Dev Jain" , "Barry Song" , "Lance Yang" , "Usama Arif" , "Kiryl Shutsemau" , "Doug Gilbert" , "James E.J. Bottomley" , "Martin K. 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Miller" , "Andreas Larsson" , "Alexander Viro" , "Christian Brauner" , "Matthew Brost" , "Joshua Hahn" , "Rakie Kim" , "Byungchul Park" , "Gregory Price" , "Ying Huang" , "Alistair Popple" , "Chris Li" , "Kairui Song" , "Kemeng Shi" , "Nhat Pham" , "Baoquan He" , "Youngjun Park" , "Johannes Weiner" , "Qi Zheng" , "Shakeel Butt" , "Axel Rasmussen" , "Yuanchu Xie" , "Wei Xu" , "Chengming Zhou" , "Michal Hocko" , "Miklos Szeredi" , "Xu Xin" From: "Zi Yan" Subject: Re: [PATCH v3 16/40] mm/vma: only allow mmap to clear VMA_MAYWRITE_BIT if kernel-owned Cc: , , , , , , , , , , , , , , , , , , , , , , , X-Mailer: aerc 0.22.0 References: <20260917-b4-mmap-prepare-vma-flag-sanify-v3-0-4583d8a23bca@kernel.org> <20260917-b4-mmap-prepare-vma-flag-sanify-v3-16-4583d8a23bca@kernel.org> In-Reply-To: <20260917-b4-mmap-prepare-vma-flag-sanify-v3-16-4583d8a23bca@kernel.org> X-ClientProxiedBy: BN9PR03CA0053.namprd03.prod.outlook.com (2603:10b6:408:fb::28) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|DS0PR12MB504719:EE_ X-MS-Office365-Filtering-Correlation-Id: 3b0d669e-e538-4760-ca84-08df1b72f1f3 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|366016|23010399003|1800799024|10067099003|11063799006|56012099006|22082099003|18002099003|4143699003|921020; 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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Thu Sep 17, 2026 at 12:22 PM EDT, Lorenzo Stoakes (ARM) wrote: > For ordinary files the only way the VMA_MAYWRITE_BIT flag is cleared is if > the underlying file is itself read-only. > > This means that mprotect() cannot mark a shared mapping of a read-only file > as read/write, as doing so would violate the read only attribute, and > permit writes. > > In general, we do not want file systems to be able to do this for > read/write files. > > Doing so would violate fundamental user expectation of file attributes and > likely break userspace. > > However, drivers pose a tricky problem here - the /dev/xxx file may be > read/write but provide access to a resource which is fundamentally > read-only. > > Therefore we must allow drivers to be able to clear VMA_MAYWRITE_BIT. > > To achieve both of these things, restrict this ability to kernel-owned > mappings as identified by vma_flags_is_kernel_owned(). > > This constrains this ability to drivers which own the mapping's contents, > whether memory-mapped I/O, kernel-allocated pages, or ordinary pages they > map themselves, and so define its semantics. > > Every in-tree mmap hook which clears VMA_MAYWRITE_BIT, some twenty sites > across drivers, filesystems and bpf, establishes a kernel-owned mapping, > with usbmon and the ALSA PCM status page converted earlier in this series > to do so. > > Note that drivers may, if they do not gate on VMA_SHARED_BIT, be able to > disable MAP_PRIVATE-file-backed mapping CoW semantics. > > This is perhaps not always intended, but we retain this capacity to > maintain existing behaviour. > > As all drivers which clear VMA_MAYWRITE_BIT establish kernel-owned > mappings, no functional change is intended. > > Signed-off-by: Lorenzo Stoakes (ARM) > --- > mm/vma.c | 5 +++++ > 1 file changed, 5 insertions(+) > Makes sense. Acked-by: Zi Yan -- Best Regards, Yan, Zi _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv