[PATCH V4 2/4] blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter

Ming Lei
2021-04-29T02:35:12Z

Grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter(), and
this way will prevent the request from being re-used when ->fn is
running. The approach is same as what we do during handling timeout.

Fix request UAF related with completion race or queue releasing:

- If one rq is referred before rq->q is frozen, then queue won't be
frozen before the request is released during iteration.

- If one rq is referred after rq->q is frozen, refcount_inc_not_zero()
will return false, and we won't iterate over this request.

However, still one request UAF not covered: refcount_inc_not_zero() may
read one freed request, and it will be handled in next patch.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq-tag.c | 22 ++++++++++++++++------
block/blk-mq.c | 14 +++++++++-----
block/blk-mq.h | 1 +
3 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2a37731e8244..9329b94a9743 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -264,6 +264,8 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
struct blk_mq_tags *tags = iter_data->tags;
bool reserved = iter_data->flags & BT_TAG_ITER_RESERVED;
struct request *rq;
+ bool ret;
+ bool iter_static_rqs = !!(iter_data->flags & BT_TAG_ITER_STATIC_RQS);

if (!reserved)
bitnr += tags->nr_reserved_tags;
@@ -272,16 +274,21 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
* We can hit rq == NULL here, because the tagging functions
* test and set the bit before assigning ->rqs[].
*/
- if (iter_data->flags & BT_TAG_ITER_STATIC_RQS)
+ if (iter_static_rqs)
rq = tags->static_rqs[bitnr];
- else
+ else {
rq = tags->rqs[bitnr];
- if (!rq)
- return true;
+ if (!rq || !refcount_inc_not_zero(&rq->ref))
+ return true;
+ }
if ((iter_data->flags & BT_TAG_ITER_STARTED) &&
!blk_mq_request_started(rq))
- return true;
- return iter_data->fn(rq, iter_data->data, reserved);
+ ret = true;
+ else
+ ret = iter_data->fn(rq, iter_data->data, reserved);
+ if (!iter_static_rqs)
+ blk_mq_put_rq_ref(rq);
+ return ret;
}

/**
@@ -348,6 +355,9 @@ void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
* indicates whether or not @rq is a reserved request. Return
* true to continue iterating tags, false to stop.
* @priv: Will be passed as second argument to @fn.
+ *
+ * We grab one request reference before calling @fn and release it after
+ * @fn returns.
*/
void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
busy_tag_iter_fn *fn, void *priv)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 927189a55575..4bd6c11bd8bc 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -909,6 +909,14 @@ static bool blk_mq_req_expired(struct request *rq, unsigned long *next)
return false;
}

+void blk_mq_put_rq_ref(struct request *rq)
+{
+ if (is_flush_rq(rq, rq->mq_hctx))
+ rq->end_io(rq, 0);
+ else if (refcount_dec_and_test(&rq->ref))
+ __blk_mq_free_request(rq);
+}
+
static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
struct request *rq, void *priv, bool reserved)
{
@@ -942,11 +950,7 @@ static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
if (blk_mq_req_expired(rq, next))
blk_mq_rq_timed_out(rq, reserved);

- if (is_flush_rq(rq, hctx))
- rq->end_io(rq, 0);
- else if (refcount_dec_and_test(&rq->ref))
- __blk_mq_free_request(rq);
-
+ blk_mq_put_rq_ref(rq);
return true;
}

diff --git a/block/blk-mq.h b/block/blk-mq.h
index 3616453ca28c..143afe42c63a 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -47,6 +47,7 @@ void blk_mq_add_to_requeue_list(struct request *rq, bool at_head,
void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list);
struct request *blk_mq_dequeue_from_ctx(struct blk_mq_hw_ctx *hctx,
struct blk_mq_ctx *start);
+void blk_mq_put_rq_ref(struct request *rq);

/*
* Internal helpers for allocating/freeing the request map
--
2.29.2