nvme_find_rq() resolves a device-supplied command id to a request with
blk_mq_tag_to_rq(), which returns whatever request last used that tag -
possibly one that is no longer in flight (freed, or never dispatched and
thus with a NULL rq->mq_hctx).  Commit e7006de6c238 ("nvme: code
command_id with a genctr for use-after-free validation") guards against
this, but its generation counter is only 4 bits wide and can be matched
by a malfunctioning or malicious device replaying command ids.  The
driver then completes a request that is not outstanding, dereferencing a
NULL rq->mq_hctx or double-completing a command:

  Oops: general protection fault ... KASAN: null-ptr-deref
  RIP: blk_mq_complete_request_remote+0xe5/0xa80 block/blk-mq.c:1319
   nvme_handle_cqe drivers/nvme/host/pci.c:1418 [inline]
   nvme_poll_cq drivers/nvme/host/pci.c:1449
   nvme_irq drivers/nvme/host/pci.c:1463

Require the request to be in flight before completing it.  The check uses
the request state, so it also covers controllers with
NVME_QUIRK_SKIP_CID_GEN.

Found by FuzzNvme(Syzkaller with FEMU fuzzing framework).

Acked-by: Sungwoo Kim <iam@sung-woo.kim>
Acked-by: Dave Tian <daveti@purdue.edu>
Acked-by: Weidong Zhu <weizhu@fiu.edu>
Signed-off-by: Chao Shi <coshi036@gmail.com>
---
 drivers/nvme/host/nvme.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 9a5f28c5103c..3a525c1dc818 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -615,6 +615,17 @@ static inline struct request *nvme_find_rq(struct blk_mq_tags *tags,
 			tag);
 		return NULL;
 	}
+	/*
+	 * blk_mq_tag_to_rq() returns whatever request last used this tag, which
+	 * may no longer be in flight if the device reports a bogus command id.
+	 * Completing it would deref a NULL rq->mq_hctx or double-complete a
+	 * command; the 4-bit genctr below only narrows the window.
+	 */
+	if (unlikely(blk_mq_rq_state(rq) != MQ_RQ_IN_FLIGHT)) {
+		dev_err(nvme_req(rq)->ctrl->device,
+			"completion for request %#x not in flight\n", tag);
+		return NULL;
+	}
 	if (unlikely(nvme_genctr_mask(nvme_req(rq)->genctr) != genctr)) {
 		dev_err(nvme_req(rq)->ctrl->device,
 			"request %#x genctr mismatch (got %#x expected %#x)\n",
-- 
2.43.0