[PATCH 5.10.y] loop: check for read-write open mode on LOOP_SET_STATUS in lo_compat_ioctl()

[Sasha Levin <sashal@kernel.org>]

From: Dominik Maier <dmnk@google.com>

[ Upstream commit 87409eedc543d288cdd49c2174f7d61eca7b8db5 ]

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>
[ sashal: Replaced BLK_OPEN_WRITE with FMODE_WRITE to match the existing fmode_t interface. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 dab31049bd4fa..a25467bb1cc74 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1850,8 +1850,10 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_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 & FMODE_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