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loop: check for read-write open mode on LOOP_SET_STATUS in lo_compat_ioctl()

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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>
This commit is contained in:
Dominik Maier 2026-10-25 21:07:40 +01:00 committed by Jens Axboe
parent 3d97e7c377
commit 87409eedc5
1 changed files with 4 additions and 2 deletions
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6
drivers/block/loop.c
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@ -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,