On 5/27/19 6:29 AM, Jan Kara wrote: > On Thu 16-05-19 14:44:07, Jens Axboe wrote: >> On 5/16/19 8:01 AM, Jan Kara wrote: >>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx >>> Signed-off-by: Jan Kara <jack@xxxxxxx> >>> --- >>> drivers/block/loop.c | 18 +++++++++++++++++- >>> 1 file changed, 17 insertions(+), 1 deletion(-) >>> >>> Hi Jens! >>> >>> What do you think about this patch? It fixes the problem but it also >>> changes user visible behavior so there are chances it breaks some >>> existing setup (although I have hard time coming up with a realistic >>> scenario where it would matter). >> >> I also have a hard time thinking about valid cases where this would be a >> problem. I think, in the end, that fixing the issue is more important >> than a potentially hypothetical use case. >> >>> Alternatively we could change getblk() code handle changing block >>> size. That would fix the particular issue syzkaller found as well but >>> I'm not sure what else is broken when block device changes while fs >>> driver is working with it. >> >> I think your solution here is saner. > > Will you pick up the patch please? I cannot find it in your tree... Thanks! Done! -- Jens Axboe
