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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=dcbte5hd; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9059B607A88C; Mon, 2 Nov 2026 14:27:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=dcbte5hd Received: by smtp.kernel.org (Postfix) with ESMTPSA id 929881F00A1A; Mon, 2 Nov 2026 14:26:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793629627; bh=vzxa9WYWKU2hIPXRh8KIDL7iPDal71uWut2R3PeQ4Y0=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=dcbte5hdW7xjuWRdXMJ9nUGRptVMxwMDxFZD2ZHCRvhATLFLbmCjO/p7ODwW6VkxD y7TF4q7FxSh3ont9POQCTTY0PwswUZV/ZqG4O8wvRMxs5F8284pF+mcGzm4MaK93NQ QpE3IepC7NC3BeKk4OMULpvVs62kxU/MrowPIKoZY6XO5y1lgBCb/ZkDGZhuIEq6NW CebUwpQ9Bu8ybMNIDU6UpP7VHTtt7ORw+YnjUGlgQRZHNtXQRbikAiiBKHiyiL/Vym Kt3ER8pjmFooN4uaNfApJD5LlOSUElugetgObusGmTVg7V6wXoTDdDCVHauOcLAVDL k6v5tGZIFoVqg== From: Christian Brauner Date: Mon, 02 Nov 2026 15:21:38 +0100 Subject: [PATCH RFC 23/38] proc: don't let NOMMU map procfs files X-Mailing-List: linux-debuggers@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261102-work-procfs-bugfixes-v1-23-59422ed3e465@kernel.org> References: <20261102-work-procfs-bugfixes-v1-0-59422ed3e465@kernel.org> In-Reply-To: <20261102-work-procfs-bugfixes-v1-0-59422ed3e465@kernel.org> To: linux-fsdevel@vger.kernel.org Cc: Linus Torvalds , Jann Horn , Josef Bacik , Chris Mason , Jeff Layton , NeilBrown , Eric Dumazet , Thomas Gleixner , Al Viro , Jonathan Corbet , Christoph Hellwig , Greg Kroah-Hartman , Joel Granados , Kees Cook , Jan Kara , Jan Kara , linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-acpi@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, linux-um@lists.infradead.org, linux-wireless@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, ntfs3@lists.linux.dev, platform-driver-x86@vger.kernel.org, coreteam@netfilter.org, kexec@lists.infradead.org, linux-pci@vger.kernel.org, linux-sound@vger.kernel.org, netfilter-devel@vger.kernel.org, dri-devel@lists.freedesktop.org, ibm-acpi-devel@lists.sourceforge.net, jfs-discussion@lists.sourceforge.net, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-cifs@vger.kernel.org, linux-fbdev@vger.kernel.org, linux-hwmon@vger.kernel.org, linux-mm@kvack.org, linux-sh@vger.kernel.org, linux-usb@vger.kernel.org, samba-technical@lists.samba.org, sparclinux@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-doc@vger.kernel.org, linux-serial@vger.kernel.org, linux-atm-general@lists.sourceforge.net, Takashi Iwai , linux-input@vger.kernel.org, linux-raid@vger.kernel.org, linux-ext4@vger.kernel.org, linux-modules@vger.kernel.org, linux-debuggers@vger.kernel.org, "Christian Brauner (Amutable)" , Chuck Lever , stable@vger.kernel.org X-Mailer: b4 0.17-dev X-Developer-Signature: v=1; a=openpgp-sha256; l=3736; i=brauner@kernel.org; h=from:subject:message-id; bh=z8wCFpT+9QBGZHOy3TWX3BTOUWtbXQrp/PYYpjal0N4=; b=owGbwMvMwCU28Zj0gdSKO4sYT6slMWS9mJPtsd9LyTjxn8etT/1L5/2vmRnbF3jUb0ZGzE+VN WutAza+7yhlYRDjYpAVU2RxaDcJl1vOU7HZKFMDZg4rE8gQBi5OAZiIwGxGhonbJLQPhS/4cP0g 78WZUsuMuwP0t0uVRT6JrMjxPpyY/4WR4cI2pcs8h/Usw6/eF8xTrDD7dyb70sRAec7Qk5mztX6 2cwAA X-Developer-Key: i=brauner@kernel.org; a=openpgp; fpr=4880B8C9BD0E5106FC070F4F7B3C391EFEA93624 Every procfs file has a ->mmap since commit 786d7e1612f0 ("Fix rmmod/read/write races in /proc entries"), whether the entry has a ->proc_mmap or not. On NOMMU validate_mmap_request() takes that to mean the file can be mapped. procfs has no ->mmap_capabilities so a regular file gets NOMMU_MAP_COPY, and a private mapping never calls ->mmap at all. Instead do_mmap() takes nommu_region_sem for writing and do_mmap_private() reads the file into the copy with kernel_read(). Reading /proc/maps takes nommu_region_sem as well: mmap(NULL, 4096, PROT_READ, MAP_PRIVATE, open("/proc/maps", O_RDONLY), 0) do_mmap() // down_write(&nommu_region_sem) -> do_mmap_private() -> kernel_read() -> seq_read_iter() -> nommu_region_list_start() // down_read() The task hangs for good, holding nommu_region_sem and its own mmap_lock, and every mmap(), munmap() and exit after that hangs behind it. /proc/maps is 0444 so any user can do that. Give the procfs files a ->mmap_capabilities on NOMMU that allows nothing. Mapping a procfs file then fails with -ENODEV, like mapping a file without ->mmap did before 786d7e1612f0. NOMMU never called ->proc_mmap() for a regular file anyway, procfs never offered NOMMU_MAP_DIRECT. Reported-by: Chris Mason Fixes: 786d7e1612f0 ("Fix rmmod/read/write races in /proc entries") Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner (Amutable) --- fs/proc/inode.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/fs/proc/inode.c b/fs/proc/inode.c index b7634f975d98..9933f03ab9fb 100644 --- a/fs/proc/inode.c +++ b/fs/proc/inode.c @@ -466,6 +466,18 @@ proc_reg_get_unmapped_area(struct file *file, unsigned long orig_addr, return rv; } +#ifndef CONFIG_MMU +/* + * NOMMU never calls ->proc_mmap() for a regular file. It reads the file into + * a private copy instead, with nommu_region_sem held for writing, which + * reading /proc/maps takes as well. Don't map procfs files at all. + */ +static unsigned int proc_reg_mmap_capabilities(struct file *file) +{ + return 0; +} +#endif + static int proc_reg_open(struct inode *inode, struct file *file) { struct proc_dir_entry *pde = PDE(inode); @@ -559,6 +571,9 @@ static const struct file_operations proc_reg_file_ops = { .unlocked_ioctl = proc_reg_unlocked_ioctl, .mmap = proc_reg_mmap, .get_unmapped_area = proc_reg_get_unmapped_area, +#ifndef CONFIG_MMU + .mmap_capabilities = proc_reg_mmap_capabilities, +#endif .open = proc_reg_open, .release = proc_reg_release, }; @@ -572,6 +587,9 @@ static const struct file_operations proc_iter_file_ops = { .unlocked_ioctl = proc_reg_unlocked_ioctl, .mmap = proc_reg_mmap, .get_unmapped_area = proc_reg_get_unmapped_area, +#ifndef CONFIG_MMU + .mmap_capabilities = proc_reg_mmap_capabilities, +#endif .open = proc_reg_open, .release = proc_reg_release, }; @@ -586,6 +604,9 @@ static const struct file_operations proc_reg_file_ops_compat = { .compat_ioctl = proc_reg_compat_ioctl, .mmap = proc_reg_mmap, .get_unmapped_area = proc_reg_get_unmapped_area, +#ifndef CONFIG_MMU + .mmap_capabilities = proc_reg_mmap_capabilities, +#endif .open = proc_reg_open, .release = proc_reg_release, }; @@ -600,6 +621,9 @@ static const struct file_operations proc_iter_file_ops_compat = { .compat_ioctl = proc_reg_compat_ioctl, .mmap = proc_reg_mmap, .get_unmapped_area = proc_reg_get_unmapped_area, +#ifndef CONFIG_MMU + .mmap_capabilities = proc_reg_mmap_capabilities, +#endif .open = proc_reg_open, .release = proc_reg_release, }; -- 2.53.0