[PATCH V6 4/4] blk-mq: clearing flush request reference in tags->rqs[]

Ming Lei
2021-05-07T14:42:51Z

Before we free request queue, clearing flush request reference in
tags->rqs[], so that potential UAF can be avoided.

Based on one patch written by David Jeffery.

Tested-by: John Garry <john.garry@huawei.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq.c | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 626b4483e006..fcd5ed79011f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2642,16 +2642,45 @@ static void blk_mq_remove_cpuhp(struct blk_mq_hw_ctx *hctx)
&hctx->cpuhp_dead);
}

+/*
+ * Before freeing hw queue, clearing the flush request reference in
+ * tags->rqs[] for avoiding potential UAF.
+ */
+static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
+ unsigned int queue_depth, struct request *flush_rq)
+{
+ int i;
+ unsigned long flags;
+
+ WARN_ON_ONCE(refcount_read(&flush_rq->ref) != 0);
+
+ for (i = 0; i < queue_depth; i++)
+ cmpxchg(&tags->rqs[i], flush_rq, NULL);
+
+ /*
+ * Wait until all pending iteration is done.
+ *
+ * Request reference is cleared and it is guaranteed to be observed
+ * after the ->lock is released.
+ */
+ spin_lock_irqsave(&tags->lock, flags);
+ spin_unlock_irqrestore(&tags->lock, flags);
+}
+
/* hctx->ctxs will be freed in queue's release handler */
static void blk_mq_exit_hctx(struct request_queue *q,
struct blk_mq_tag_set *set,
struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
{
+ struct request *flush_rq = hctx->fq->flush_rq;
+
if (blk_mq_hw_queue_mapped(hctx))
blk_mq_tag_idle(hctx);

+ blk_mq_clear_flush_rq_mapping(set->tags[hctx_idx],
+ set->queue_depth, flush_rq);
if (set->ops->exit_request)
- set->ops->exit_request(set, hctx->fq->flush_rq, hctx_idx);
+ set->ops->exit_request(set, flush_rq, hctx_idx);

if (set->ops->exit_hctx)
set->ops->exit_hctx(hctx, hctx_idx);
--
2.29.2