Re: [PATCH] bfq: Check if bfqq is NULL in bfq_insert_request

[Guenter Roeck]

On Mon, Jul 22, 2019 at 10:30:48AM -0700, Guenter Roeck wrote:
> In bfq_insert_request(), bfqq is initialized with:
> 	bfqq = bfq_init_rq(rq);
> In bfq_init_rq(), we find:
> 	if (unlikely(!rq->elv.icq))
> 		return NULL;
> Indeed, rq->elv.icq can be NULL if the memory allocation in
> create_task_io_context() failed.
> 

The above function should have been ioc_create_icq(), sorry.

Guenter

> A comment in bfq_insert_request() suggests that bfqq is supposed to be
> non-NULL if 'at_head || blk_rq_is_passthrough(rq)' is false. Yet, as
> debugging and practical experience shows, this is not the case in the
> above situation.
> 
> This results in the following crash.
> 
> Unable to handle kernel NULL pointer dereference
> 	at virtual address 00000000000001b0
> ...
> Call trace:
>  bfq_setup_cooperator+0x44/0x134
>  bfq_insert_requests+0x10c/0x630
>  blk_mq_sched_insert_requests+0x60/0xb4
>  blk_mq_flush_plug_list+0x290/0x2d4
>  blk_flush_plug_list+0xe0/0x230
>  blk_finish_plug+0x30/0x40
>  generic_writepages+0x60/0x94
>  blkdev_writepages+0x24/0x30
>  do_writepages+0x74/0xac
>  __filemap_fdatawrite_range+0x94/0xc8
>  file_write_and_wait_range+0x44/0xa0
>  blkdev_fsync+0x38/0x68
>  vfs_fsync_range+0x68/0x80
>  do_fsync+0x44/0x80
> 
> The problem is relatively easy to reproduce by running an image with
> failslab enabled, such as with:
> 
> cd /sys/kernel/debug/failslab
> echo 10 > probability
> echo 300 > times
> 
> Avoid the problem by checking if bfqq is NULL before using it. With the
> NULL check in place, requests with missing io context are queued
> immediately, and the crash is no longer seen.
> 
> Fixes: 18e5a57d79878 ("block, bfq: postpone rq preparation to insert or merge")
> Reported-by: Hsin-Yi Wang  <hsinyi@xxxxxxxxxx>
> Cc: Hsin-Yi Wang <hsinyi@xxxxxxxxxx>
> Cc: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> Cc: Doug Anderson <dianders@xxxxxxxxxxxx>
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> ---
>  block/bfq-iosched.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 72860325245a..56f3f4227010 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -5417,7 +5417,7 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  
>  	spin_lock_irq(&bfqd->lock);
>  	bfqq = bfq_init_rq(rq);
> -	if (at_head || blk_rq_is_passthrough(rq)) {
> +	if (!bfqq || at_head || blk_rq_is_passthrough(rq)) {
>  		if (at_head)
>  			list_add(&rq->queuelist, &bfqd->dispatch);
>  		else
> -- 
> 2.7.4
>