block: blk_init_allocated_queue() set q->fq as NULL in the fail case (CVE-2018-20856)

PD#SWPL-26439

commit 54648cf1ec upstream.

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().

Change-Id: I8f328135c35326c60be07ec23926720cc3cb9ccd
Fixes: commit 7c94e1c157 ("block: introduce blk_flush_queue to drive flush machinery")
Cc: <stable@vger.kernel.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: xiao jin <jin.xiao@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[groeck: backport to v4.4.y/v4.9.y (context change)]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

block/blk-core.c

@@ -886,6 +886,7 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
 
 fail:
 	blk_free_flush_queue(q->fq);
+	q->fq = NULL;
 	return NULL;
 }
 EXPORT_SYMBOL(blk_init_allocated_queue);