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Tue, 10 Nov 2026 09:15:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794302164; bh=2Uxnm4Cd0mizACq2HNobDQRQcgrPnhEO2KA80+bveeM=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=ba3Pyp5OpjsLWNS18IN6KJmuJFLn691WGOW4O+tQFCTWln2eXYVZ3JE1wBRhcmoW6 hLpwpT4jz67cKW4CJB9uIhzhbMhfgiFHlr2CZK53VaqCwK5HM1YqSaiFHCScsFMBkn Q0OzOCwsepsGyP/WRgtRo4+3FcZl8eUfyuWZ7KuL8XDVDG6tcU8rd25pxcEBVszf1y XJvR4c13VrmJU77uH8rLkN68FB+fLrhpSvF6buW7PxoKBBbfZNWSogmUA09LLIYVWc o8T8B1sqCgT2o3NyN+rjS4hlvf0pw7ESlo7MI8nYdimYq23hA2cdZ9j7+asrP+da9o Z/f8j2gHp/WZg== From: Christian Brauner Date: Tue, 10 Nov 2026 10:14:17 +0100 Subject: [PATCH RFC 01/41] loop: refuse backing files that aren't open for reading X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , Precedence: list X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261110-work-loop-pseudo-v1-1-4c998976c6c7@kernel.org> References: <20261110-work-loop-pseudo-v1-0-4c998976c6c7@kernel.org> In-Reply-To: <20261110-work-loop-pseudo-v1-0-4c998976c6c7@kernel.org> To: linux-fsdevel@vger.kernel.org Cc: Jann Horn , Chris Mason , Josef Bacik , Christoph Hellwig , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, Alexander Viro , Jan Kara , linux-scsi@vger.kernel.org, target-devel@vger.kernel.org, driver-core@lists.linux.dev, linux-mm@kvack.org, bpf@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, apparmor@lists.ubuntu.com, linux-nfs@vger.kernel.org, netdev@vger.kernel.org, fuse-devel@lists.linux.dev, linux-efi@vger.kernel.org, xen-devel@lists.xenproject.org, ocfs2-devel@lists.linux.dev, linux-s390@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-usb@vger.kernel.org, Amir Goldstein , linux-erofs@lists.ozlabs.org, linux-kselftest@vger.kernel.org, "Christian Brauner (Amutable)" , stable@vger.kernel.org X-Mailer: b4 0.17-dev-ce4c1 X-Developer-Signature: v=1; a=openpgp-sha256; l=1688; i=brauner@kernel.org; h=from:subject:message-id; bh=k5rFTn5qwSY0P1NIV6SDcJMZmh5p3efbZZ8FHYDs+to=; b=owGbwMvMwCU28Zj0gdSKO4sYT6slMWR9ejBVvfNKBs+f5kIhpoaev8LtD4oXZ1xL0i19ll21p i5fZsmkjlIWBjEuBlkxRRaHdpNwueU8FZuNMjVg5rAygQxh4OIUgInc3s/I0OHqXaH2k1fp545n UtNUpkUwfzq4/7uEmpjnzfcNZ9bvT2JkeCygm5CxsPhqWqRgSqf6D/8dk2afLnMX1LJ/rjh/3zZ BPgA= X-Developer-Key: i=brauner@kernel.org; a=openpgp; fpr=4880B8C9BD0E5106FC070F4F7B3C391EFEA93624 X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 loop_configure() takes any file as long as it implements ->read_iter(). Loop itself never checked whether the file was actually opened for reading. Buffered I/O went through vfs_iter_read() which refuses files without FMODE_READ. Direct I/O has called ->read_iter() of the backing file directly ever since loop grew support for it. Since commit f2fed441c69b ("loop: stop using vfs_iter_{read,write} for buffered I/O") buffered I/O calls ->read_iter() directly as well. That commit was about O_DIRECT handling and the FMODE_READ check went away as a side effect. So whoever can set up a loop device can read a file it was only allowed to open for writing. Open the file O_WRONLY, configure a loop device with it and read the contents through the loop device. Refuse backing files that aren't open for reading. Loop always reads from its backing file so a write-only backing file never made sense in the first place. Fixes: bc07c10a3603 ("block: loop: support DIO & AIO") Fixes: f2fed441c69b ("loop: stop using vfs_iter_{read,write} for buffered I/O") Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner (Amutable) --- drivers/block/loop.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index a3325ceab484..c96e35c3ef42 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -528,6 +528,9 @@ static void loop_assign_backing_file(struct loop_device *lo, struct file *file) static int loop_check_backing_file(struct file *file) { + if (!(file->f_mode & FMODE_READ)) + return -EBADF; + if (!file->f_op->read_iter) return -EINVAL; -- 2.53.0