Re: [PATCH v3] block/loop: Serialize ioctl operations.

From: Tetsuo Handa
Date: Mon Jun 04 2018 - 07:19:15 EST

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]
> [2]
> 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(-)