From: Jan Kara jack@suse.cz
[ Upstream commit 33ec3e53e7b1869d7851e59e126bdb0fe0bd1982 ]
Loop module allows calling LOOP_SET_FD while there are other openers of the loop device. Even exclusive ones. This can lead to weird consequences such as kernel deadlocks like:
mount_bdev() lo_ioctl() udf_fill_super() udf_load_vrs() sb_set_blocksize() - sets desired block size B udf_tread() sb_bread() __bread_gfp(bdev, block, B) loop_set_fd() set_blocksize() - now __getblk_slow() indefinitely loops because B != bdev block size
Fix the problem by disallowing LOOP_SET_FD ioctl when there are exclusive openers of a loop device.
[Deliberately chosen not to CC stable as a user with priviledges to trigger this race has other means of taking the system down and this has a potential of breaking some weird userspace setup]
Reported-and-tested-by: syzbot+10007d66ca02b08f0e60@syzkaller.appspotmail.com Signed-off-by: Jan Kara jack@suse.cz Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/block/loop.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c index f1e63eb7cbca..a443910f5d6f 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -920,9 +920,20 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode, if (!file) goto out;
+ /* + * If we don't hold exclusive handle for the device, upgrade to it + * here to avoid changing device under exclusive owner. + */ + if (!(mode & FMODE_EXCL)) { + bdgrab(bdev); + error = blkdev_get(bdev, mode | FMODE_EXCL, loop_set_fd); + if (error) + goto out_putf; + } + error = mutex_lock_killable(&loop_ctl_mutex); if (error) - goto out_putf; + goto out_bdev;
error = -EBUSY; if (lo->lo_state != Lo_unbound) @@ -986,10 +997,15 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode, mutex_unlock(&loop_ctl_mutex); if (partscan) loop_reread_partitions(lo, bdev); + if (!(mode & FMODE_EXCL)) + blkdev_put(bdev, mode | FMODE_EXCL); return 0;
out_unlock: mutex_unlock(&loop_ctl_mutex); +out_bdev: + if (!(mode & FMODE_EXCL)) + blkdev_put(bdev, mode | FMODE_EXCL); out_putf: fput(file); out: