Patch "bsg-lib: fix use-after-free under memory-pressure" has been added to the 4.13-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.13-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bsg-lib-fix-use-after-free-under-memory-pressure.patch
and it can be found in the queue-4.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Benjamin Block <[email protected]>
Date: Tue, 3 Oct 2017 12:48:37 +0200
Subject: bsg-lib: fix use-after-free under memory-pressure

From: Benjamin Block <[email protected]>

commit eab40cf336065e8d765e006b81ff48c5c114b365 upstream.

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.

---
 block/bsg-lib.c |   27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

@@ -207,20 +207,34 @@ static int bsg_init_rq(struct request_qu
 	struct bsg_job *job = blk_mq_rq_to_pdu(req);
 	struct scsi_request *sreq = &job->sreq;
 
+	/* called right after the request is allocated for the request_queue */
+
+	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;
-	sreq->sense = kzalloc(sreq->sense_len, gfp);
-	if (!sreq->sense)
-		return -ENOMEM;
 
 	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)

@@ -250,6 +264,7 @@ struct request_queue *bsg_setup_queue(st
 	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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.13/bsg-lib-fix-use-after-free-under-memory-pressure.patch