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

[Bob Liu]

On 7/23/19 4:29 AM, Guenter Roeck wrote:
> 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.
>>


What about other place use blk_init_rq()?
E.g in bfq_request_merged():
1897                 struct bfq_queue *bfqq = bfq_init_rq(req);
1898                 struct bfq_data *bfqd = bfqq->bfqd;

Which may have same Null-pointer bug?

-Bob

>> 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
>>