bsg-lib: fix use-after-free under memory-pressure

When under memory-pressure it is possible that the mempool which backs
the 'struct request_queue' will make use of up to BLKDEV_MIN_RQ count
emergency buffers - in case it can't get a regular allocation. These
buffers are preallocated and once they are also used, they are
re-supplied with old finished requests from the same request_queue (see
mempool_free()).

The bug is, when re-supplying the emergency pool, the old requests are
not again ran through the callback mempool_t->alloc(), and thus also not
through the callback bsg_init_rq(). Thus we skip initialization, and
while the sense-buffer still should be good, scsi_request->cmd might
have become to be an invalid pointer in the meantime. When the request
is initialized in bsg.c, and the user's CDB is larger than BLK_MAX_CDB,
bsg will replace it with a custom allocated buffer, which is freed when
the user's command is finished, thus it dangles afterwards. When next a
command is sent by the user that has a smaller/similar CDB as
BLK_MAX_CDB, bsg will assume that scsi_request->cmd is backed by
scsi_request->__cmd, will not make a custom allocation, and write into
undefined memory.

Fix this by splitting bsg_init_rq() into two functions:
 - bsg_init_rq() is changed to only do the allocation of the
   sense-buffer, which is used to back the bsg job's reply buffer. This
   pointer should never change during the lifetime of a scsi_request, so
   it doesn't need re-initialization.
 - bsg_initialize_rq() is a new function that makes use of
   'struct request_queue's initialize_rq_fn callback (which was
   introduced in v4.12). This is always called before the request is
   given out via blk_get_request(). This function does the remaining
   initialization that was previously done in bsg_init_rq(), and will
   also do it when the request is taken from the emergency-pool of the
   backing mempool.

Fixes: 50b4d48552 ("bsg-lib: fix kernel panic resulting from missing allocation of reply-buffer")
Cc: <stable@vger.kernel.org> # 4.11+
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bsg-lib.c

@@ -207,20 +207,34 @@ static int bsg_init_rq(struct request_queue *q, struct request *req, gfp_t gfp)
 	struct bsg_job *job = blk_mq_rq_to_pdu(req);
 	struct scsi_request *sreq = &job->sreq;
 
-	memset(job, 0, sizeof(*job));
+	/* called right after the request is allocated for the request_queue */
 
-	scsi_req_init(sreq);
-	sreq->sense_len = SCSI_SENSE_BUFFERSIZE;
-	sreq->sense = kzalloc(sreq->sense_len, gfp);
+	sreq->sense = kzalloc(SCSI_SENSE_BUFFERSIZE, gfp);
 	if (!sreq->sense)
 		return -ENOMEM;
 
+	return 0;
+}
+
+static void bsg_initialize_rq(struct request *req)
+{
+	struct bsg_job *job = blk_mq_rq_to_pdu(req);
+	struct scsi_request *sreq = &job->sreq;
+	void *sense = sreq->sense;
+
+	/* called right before the request is given to the request_queue user */
+
+	memset(job, 0, sizeof(*job));
+
+	scsi_req_init(sreq);
+
+	sreq->sense = sense;
+	sreq->sense_len = SCSI_SENSE_BUFFERSIZE;
+
 	job->req = req;
-	job->reply = sreq->sense;
+	job->reply = sense;
 	job->reply_len = sreq->sense_len;
 	job->dd_data = job + 1;
-
-	return 0;
 }
 
 static void bsg_exit_rq(struct request_queue *q, struct request *req)
@@ -251,6 +265,7 @@ struct request_queue *bsg_setup_queue(struct device *dev, const char *name,
 	q->cmd_size = sizeof(struct bsg_job) + dd_job_size;
 	q->init_rq_fn = bsg_init_rq;
 	q->exit_rq_fn = bsg_exit_rq;
+	q->initialize_rq_fn = bsg_initialize_rq;
 	q->request_fn = bsg_request_fn;
 
 	ret = blk_init_allocated_queue(q);