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 A386A545D9A; Tue, 8 Sep 2026 20:09:36 +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=1788898178; cv=none; b=A9nD+D6JezWOitbNyyvj1EHwI5g59xWz2yPjMhdzsDjsFSItPUvqXdk+DC/uRIrGs9LJZQSuVq85ShRXrAzuklJTUPkaUhCsJBeu8TJ24g5mwL5Z+KuupTiMnlcCFJvd5+q44tcT+pdtk4mfjc64UZxSeZORwzBwl37k+LQuEUc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788898178; c=relaxed/simple; bh=RGYbj2HHlqxNjIH/BiZsGGf9u4x+neMkFtHuPWWBclU=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=uI1eETXrjaeFPWzgZxcuw/yT48xX0C3qhjPxi30ig/AYUpmbBRQn84VSToO3ypaETqNojOmHi3qiBVojj7jvFEkANbPbm+hoZVzizXO7hxFpHUKIg/EgbFgKyTVpIZK7Iby6KByw3LN7MJoVKCWPadNDsM4T6ydSAzjPGIoUCBs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bOh9qaVC; 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="bOh9qaVC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 085C11F00A3A; Tue, 8 Sep 2026 20:09:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788898176; bh=p6dXD729ADCogJ3R/2wo51+NAzY7DqxT9oUL1eYTac8=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=bOh9qaVCnrZTWRlq9PaX4TcWsfcuLV7Sk55MPnRysjd8DAKGgc5dbOGtlcX+R2Cct etBvSqJ826/6aRGiGtQTdPbMl+HUHI65BgItFp7dB3nEU6+P4nEKEQuNokScxDAGQF 1FaBk52Dtsu0Sj4lK3UEIwxTGPy3xPZsEJhkMYRkKSFy9wGxZ8EyeXQaET+rodI8WW O/YNIb3WpWuRKRCra6d2SyqzYF+1G2MDLogKznQgseOC4k+u/CpmskHe9Vw88IcYKe RfnYBGrEddugW7tvStnErBL9GNi//B0nMtdD+kFUuBn48Nqiv9Gl8vTlzDJSrGw4yP Z9voucWtUuFqg== From: "Lorenzo Stoakes (ARM)" Date: Tue, 08 Sep 2026 21:01:19 +0100 Subject: [PATCH 15/39] mm/vma: only allow mmap to clear VMA_MAYWRITE_BIT if kernel-owned Precedence: bulk X-Mailing-List: linux-fbdev@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: <20260908-b4-mmap-prepare-vma-flag-sanify-v1-15-dacf19cce22b@kernel.org> References: <20260908-b4-mmap-prepare-vma-flag-sanify-v1-0-dacf19cce22b@kernel.org> In-Reply-To: <20260908-b4-mmap-prepare-vma-flag-sanify-v1-0-dacf19cce22b@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 , Xu Xin , Chengming Zhou , Michal Hocko , Miklos Szeredi 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=2245; i=ljs@kernel.org; h=from:subject:message-id; bh=RGYbj2HHlqxNjIH/BiZsGGf9u4x+neMkFtHuPWWBclU=; b=owGbwMvMwCV2fu7ZrsZH9SKMp9WSGLIWZG6KibjG1FkTM+mYgfPWohkScT/Xb9Je2BYwaa8ks yHfY4OSjlIWBjEuBlkxRZbnX8T3B4mEzeu84O8GM4eVCWQIAxenAEzkBg8jwwWjXZ+PPozePoEj 91kM1/nIHiO2iOq9T7OszDhdPvc8Xcvw33PlsoVL3+0qfv9hsV1yu1KidJlsg/7GnOnrzLas9Ck 5wA8A 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 9e45fc83f74c..526428753218 100644 --- a/mm/vma.c +++ b/mm/vma.c @@ -2780,6 +2780,11 @@ static int mmap_validate(unsigned long prev_start, 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