We find the memory use-after-free issue in __blk_drain_queue()
on the kernel 4.14. After read the latest kernel 4.18-rc6 we
think it has the same problem.

Memory is allocated for q->fq in the blk_init_allocated_queue().
If the elevator init function called with error return, it will
run into the fail case to free the q->fq.

Then the __blk_drain_queue() uses the same memory after the free
of the q->fq, it will lead to the unpredictable event.

The patch is to set q->fq as NULL in the fail case of
blk_init_allocated_queue().

Fixes: commit 7c94e1c157a2 ("block: introduce blk_flush_queue to drive flush machinery")
Signed-off-by: xiao jin <redacted>
Cc: Ming Lei <redacted>
Cc: Bart Van Assche <redacted>
Cc: <redacted>
---
 block/blk-core.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index b888175..52635e2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -1072,6 +1072,7 @@ int blk_init_allocated_queue(struct request_queue *q)
                q->exit_rq_fn(q, q->fq->flush_rq);
 out_free_flush_queue:
        blk_free_flush_queue(q->fq);
+       q->fq = NULL;
        return -ENOMEM;
 }
 EXPORT_SYMBOL(blk_init_allocated_queue);
--

1.7.9.5