linux-kernel - [PATCH] blk-mq: Avoid race condition with uninitialized requests

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Date:	Thu, 18 Sep 2014 11:04:31 +0200
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	tom.leiming@...il.com, axboe@...nel.dk
Cc:	kvm@...r.kernel.org, stable@...r.kernel.org,
	borntraeger@...ibm.com, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org, mst@...hat.com,
	dahi@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: [PATCH] blk-mq: Avoid race condition with uninitialized requests

This patch should fix the bug reported in https://lkml.org/lkml/2014/9/11/249.

We have to initialize at least the atomic_flags and the cmd_flags when
allocating storage for the requests.

Otherwise blk_mq_timeout_check() might dereference uninitialized pointers when
racing with the creation of a request.

Also move the reset of cmd_flags for the initializing code to the point where a
request is freed. So we will never end up with pending flush request indicators
that might trigger dereferences of invalid pointers in blk_mq_timeout_check().

Cc: stable@...r.kernel.org
Signed-off-by: David Hildenbrand <dahi@...ux.vnet.ibm.com>
---
 block/blk-mq.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 383ea0c..eed6340 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -203,7 +203,6 @@ __blk_mq_alloc_request(struct blk_mq_alloc_data *data, int rw)
 	if (tag != BLK_MQ_TAG_FAIL) {
 		rq = data->hctx->tags->rqs[tag];
 
-		rq->cmd_flags = 0;
 		if (blk_mq_tag_busy(data->hctx)) {
 			rq->cmd_flags = REQ_MQ_INFLIGHT;
 			atomic_inc(&data->hctx->nr_active);
@@ -258,6 +257,7 @@ static void __blk_mq_free_request(struct blk_mq_hw_ctx *hctx,
 
 	if (rq->cmd_flags & REQ_MQ_INFLIGHT)
 		atomic_dec(&hctx->nr_active);
+	rq->cmd_flags = 0;
 
 	clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
 	blk_mq_put_tag(hctx, tag, &ctx->last_tag);
@@ -1404,6 +1404,8 @@ static struct blk_mq_tags *blk_mq_init_rq_map(struct blk_mq_tag_set *set,
 		left -= to_do * rq_size;
 		for (j = 0; j < to_do; j++) {
 			tags->rqs[i] = p;
+			tags->rqs[i]->atomic_flags = 0;
+			tags->rqs[i]->cmd_flags = 0;
 			if (set->ops->init_request) {
 				if (set->ops->init_request(set->driver_data,
 						tags->rqs[i], hctx_idx, i,
-- 
1.8.5.5

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