blk-mq: fix null pointer dereference in blk_mq_clear_rq_mapping()

In the Linux kernel, the following vulnerability has been resolved:

blk-mq: fix null pointer dereference in blk_mq_clear_rq_mapping()

Our syzkaller report a null pointer dereference, root cause is

following:

__blk_mq_alloc_map_and_rqs

set->tags[hctx_idx] = blk_mq_alloc_map_and_rqs

blk_mq_alloc_map_and_rqs

blk_mq_alloc_rqs

// failed due to oom

alloc_pages_node

// set->tags[hctx_idx] is still NULL

blk_mq_free_rqs

drv_tags = set->tags[hctx_idx];

// null pointer dereference is triggered

blk_mq_clear_rq_mapping(drv_tags, ...)

This is because commit 63064be150e4 ("blk-mq:

Add blk_mq_alloc_map_and_rqs()") merged the two steps:

1) set->tags[hctx_idx] = blk_mq_alloc_rq_map()

2) blk_mq_alloc_rqs(..., set->tags[hctx_idx])

into one step:

set->tags[hctx_idx] = blk_mq_alloc_map_and_rqs()

Since tags is not initialized yet in this case, fix the problem by

checking if tags is NULL pointer in blk_mq_clear_rq_mapping().