From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6873F50E5AB; Thu, 17 Sep 2026 16:31:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789662670; cv=none; b=mr2lE9wdasA6DN4ilaynFFnbeOLSfUxBMHaEFnAm0HyjUMpwOWhCrhhXHC8Zv1NB8dbOrJWmB4vpkHhZu7BeqC/V3P6HWpDQEUujFeY0oDbpnvlDZBYtA4QJXdqZjdj76pZTpX6vBs1Cg/Fi4UhwHQ4R85GNd8YogWH5MiKFYLQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789662670; c=relaxed/simple; bh=AQdN68/spfXtpTeKBaZ+Me0gdfgYIcDdx+d6akTD8vU=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=AE8zbzPb1chiO7KfNqJgMxZfY9ZwIDDNDx/WJOaXXDWz1ZMU9sw76pZeCcWEnxvuooNuugnX7IdU47IzvheET+uvl7wUa3dKB14ZL+QkDF1xhUfNp92ozy9+bz7MYoSoE5Xj1wEdj4qvRUlYGTT3et9cLYHYTc3xVzpkLy4Xzco= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=V0PXlPCx; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="V0PXlPCx" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 474621F00898; Thu, 17 Sep 2026 16:30:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789662669; bh=L7jaJ3XNIQiKLT5qkSlMCmmbkCuTcTW6Guk4wu+FJrg=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=V0PXlPCx7005iUwQIzRn8WrFZyyFxtVLov03zt/j4Cb9KmBmrLJgrmIpFAyfapLrd H+8UUkC5mv9ThnEPdrIeBvRjIZCRkR6q8+DWj+26jr/U/1bc099Dt09f3XbstGf0tB GPF/qledlC6JuLnn7ZKiMvsnyi5RO8rzGZHKayPV0lImzjxMdDbwLfhcGBqXR6uGVW sVoxSfdhF2FyDVHTK+GQKY24DJ87K2yGos/VWGWHmcjL23ivX7hPCN7F87RaumZo8V n08M5BJ2yk7qRbIrAGbhKLJVnMQk6AD7IM19ETyOl35+W21+u+ROFzCF9UfcNa1wD3 hUJk1gDPbvIzQ== From: "Lorenzo Stoakes (ARM)" Date: Thu, 17 Sep 2026 17:22:25 +0100 Subject: [PATCH v3 16/40] mm/vma: only allow mmap to clear VMA_MAYWRITE_BIT if kernel-owned Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260917-b4-mmap-prepare-vma-flag-sanify-v3-16-4583d8a23bca@kernel.org> References: <20260917-b4-mmap-prepare-vma-flag-sanify-v3-0-4583d8a23bca@kernel.org> In-Reply-To: <20260917-b4-mmap-prepare-vma-flag-sanify-v3-0-4583d8a23bca@kernel.org> To: Andrew Morton , "Liam R. 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 , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Doug Gilbert , "James E.J. Bottomley" , "Martin K. Petersen" , Jaya Kumar , Simona Vetter , Helge Deller , Sebastian Reichel , John Hubbard , Peter Xu , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Rik van Riel , Harry Yoo , Juri Lelli , Vincent Guittot , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Arnd Bergmann , Muchun Song , Oscar Salvador , "Matthew Wilcox (Oracle)" , Jan Kara , Marc Zyngier , Oliver Upton , Catalin Marinas , Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , "David S. 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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-usb@vger.kernel.org, linux-rdma@vger.kernel.org, selinux@vger.kernel.org, linux-sound@vger.kernel.org, bpf@vger.kernel.org, linux-scsi@vger.kernel.org, linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, fuse-devel@lists.linux.dev, "Lorenzo Stoakes (ARM)" X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=2269; i=ljs@kernel.org; h=from:subject:message-id; bh=AQdN68/spfXtpTeKBaZ+Me0gdfgYIcDdx+d6akTD8vU=; b=owGbwMvMwCV2fu7ZrsZH9SKMp9WSGLLWCB+U/XmtbGsK473bjdP6GHabnKk7IeVwVcs4JsDOL zjlgPHCjlIWBjEuBlkxRZbnX8T3B4mEzeu84O8GM4eVCWQIAxenAExkki3DP3WRy9bveDdY/uc7 LPb+sUuFZsGHpirJ2GS33v1dZbxT9BgZttZGce3vv72WZ5veMwbrRweunnzCVMCUcfVzwm2X7hc tjAA= X-Developer-Key: i=ljs@kernel.org; a=openpgp; fpr=E7F417BF5214569E89D04F46CF9DCD8A81E27F14 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(+) diff --git a/mm/vma.c b/mm/vma.c index 62f2ce1ad5a1..1dec10946918 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -2795,6 +2795,11 @@ static int mmap_validate(unsigned long prev_start, unsigned long prev_end, if (WARN_ON_ONCE(!was_maywrite && is_maywrite)) return -EINVAL; + /* Only kernel-owned mappings may clear VMA_MAYWRITE_BIT. */ + if (!vma_flags_is_kernel_owned(curr_flags) && + WARN_ON_ONCE(was_maywrite && !is_maywrite)) + return -EINVAL; + return mmap_validate_vma_flags(curr_flags); } -- 2.55.0