FAILED: patch "[PATCH] loop: check for read-write open mode on LOOP_SET_STATUS in lo_compat_ioctl()" failed to apply to 6.1-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 87409eedc543d288cdd49c2174f7d61eca7b8db5
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107055527-failed-6.1-87409eedc543@kernel.org' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 87409eedc543d288cdd49c2174f7d61eca7b8db5 Mon Sep 17 00:00:00 2001
From: Dominik Maier <dmnk@google.com>
Date: Sun, 25 Oct 2026 21:07:40 +0100
Subject: [PATCH] loop: check for read-write open mode on LOOP_SET_STATUS in
 lo_compat_ioctl()

lo_ioctl() checks for (mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN)
before allowing LOOP_SET_STATUS or LOOP_SET_STATUS64, but not
lo_compat_ioctl(), the handler used by 32-bit user space on 64-bit
kernels.

As a result, on 64-bit kernels with CONFIG_COMPAT=y, a 32-bit
unprivileged user-space process holding a read-only (O_RDONLY) file
descriptor to an active /dev/loopN device can issue a compat
LOOP_SET_STATUS ioctl to modify lo_offset, clear lo_sizelimit to 0
(exposing backing-file data past the configured limit), or alter
lo_flags.

Enforce the same (mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN) check
on LOOP_SET_STATUS in lo_compat_ioctl().

Cc: stable@vger.kernel.org
Assisted-by: ARTIST LLM
Signed-off-by: Dominik Maier <dmnk@google.com>
Reviewed-by: Bart Van Assche bvanassche@acm.org
Link: https://patch.msgid.link/20261025200740.2064379-1-dmnk@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 drivers/block/loop.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index aabaaacdb4fab..a3325ceab484a 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1604,8 +1604,10 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
 
 	switch(cmd) {
 	case LOOP_SET_STATUS:
-		err = loop_set_status_compat(lo,
-			     (const struct compat_loop_info __user *)arg);
+		err = -EPERM;
+		if ((mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN))
+			err = loop_set_status_compat(lo,
+				(const struct compat_loop_info __user *)arg);
 		break;
 	case LOOP_GET_STATUS:
 		err = loop_get_status_compat(lo,
-- 
2.53.0