This problem was already ignored for 8 months. Unless we boost priority, this problem will be ignored for years. Jens, can we test this patch? On 2018/05/26 10:16, Tetsuo Handa wrote: > syzbot is reporting NULL pointer dereference [1] which is caused by > race condition between ioctl(loop_fd, LOOP_CLR_FD, 0) versus > ioctl(other_loop_fd, LOOP_SET_FD, loop_fd) due to traversing other > loop devices without holding corresponding locks. > > syzbot is also reporting circular locking dependency between bdev->bd_mutex > and lo->lo_ctl_mutex [2] which is caused by calling blkdev_reread_part() > with lock held. > > Since ioctl() request on loop devices is not frequent operation, we don't > need fine grained locking. Let's use global lock and simplify the locking > order. > > Strategy is that the global lock is held upon entry of ioctl() request, > and release it before either starting operations which might deadlock or > leaving ioctl() request. After the global lock is released, current thread > no longer uses "struct loop_device" memory because it might be modified > by next ioctl() request which was waiting for current ioctl() request. > > In order to enforce this strategy, this patch inversed > loop_reread_partitions() and loop_unprepare_queue() in loop_clr_fd(). > I don't know whether it breaks something, but I don't have testcases. > > Since this patch serializes using global lock, race bugs should no longer > exist. Thus, it will be easy to test whether this patch broke something. > > [1] https://syzkaller.appspot.com/bug?id=f3cfe26e785d85f9ee259f385515291d21bd80a3 > [2] https://syzkaller.appspot.com/bug?id=bf154052f0eea4bc7712499e4569505907d15889 > > Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> > Reported-by: syzbot <syzbot+bf89c128e05dd6c62523@xxxxxxxxxxxxxxxxxxxxxxxxx> > Reported-by: syzbot <syzbot+4684a000d5abdade83fac55b1e7d1f935ef1936e@xxxxxxxxxxxxxxxxxxxxxxxxx> > Cc: Jens Axboe <axboe@xxxxxxxxx> > --- > drivers/block/loop.c | 231 ++++++++++++++++++++++++++++----------------------- > drivers/block/loop.h | 1 - > 2 files changed, 128 insertions(+), 104 deletions(-)
