Re: [PATCH v2 1/1] bsg-lib: fix use-after-free under memory-pressure

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig 

[PATCH v2 1/1] bsg-lib: fix use-after-free under memory-pressure

[Benjamin Block]

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: 50b4d485528d ("bsg-lib: fix kernel panic resulting from missing 
allocation of reply-buffer")
Cc:  # 4.11+
Reviewed-by: Christoph Hellwig 
Signed-off-by: Benjamin Block 
---

Notes:
I did test this on zFCP with FC CT commands send via the ioctl() and
write() system-call. That did work fine. But I would very much
appreciate if anyone could run this against an other HBA or even an
other implementer of bsg-lib, such as now SAS, because I have no access
to such hardware here.

This should make no difference to the normal cases - where each request
is allocated via slab - with- or without this patch; if I didn't miss
anything. Only the order is a bit mixed up - the memset is done after
the sense-allocation, so I have to buffer the sense-pointer for that.
But otherwise there is no difference I am aware of, so it should behave
the same (does for me).

I could not reproduce the memory-pressure case here in the lab.. I
don't see any reason why it should work now, but I am open to
suggestions :)

Beste Grüße / Best regards,
  - Benjamin Block

Changes in v2:
 - Renamed function names to reflect the function-pointer names in
   request_queue

 block/bsg-lib.c | 29 ++---
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/block/bsg-lib.c b/block/bsg-lib.c
index c82408c7cc3c..ff5a158b22fa 100644
--- a/block/bsg-lib.c
+++ b/block/bsg-lib.c
@@ -208,20 +208,34 @@ 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 = >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 = >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)
@@ -252,6 +266,7 @@ 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 =