[PATCH V6 2/4] blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter

Ming Lei
2021-05-07T14:42:38Z

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 use-after-free(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.

Tested-by: John Garry <john.garry@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq-tag.c | 44 +++++++++++++++++++++++++++++++++-----------
block/blk-mq.c | 14 +++++++++-----
block/blk-mq.h | 1 +
3 files changed, 43 insertions(+), 16 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2a37731e8244..544edf2c56a5 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -199,6 +199,16 @@ struct bt_iter_data {
bool reserved;
};

+static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
+ unsigned int bitnr)
+{
+ struct request *rq = tags->rqs[bitnr];
+
+ if (!rq || !refcount_inc_not_zero(&rq->ref))
+ return NULL;
+ return rq;
+}
+
static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
{
struct bt_iter_data *iter_data = data;
@@ -206,18 +216,22 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
struct blk_mq_tags *tags = hctx->tags;
bool reserved = iter_data->reserved;
struct request *rq;
+ bool ret = true;

if (!reserved)
bitnr += tags->nr_reserved_tags;
- rq = tags->rqs[bitnr];
-
/*
* We can hit rq == NULL here, because the tagging functions
* test and set the bit before assigning ->rqs[].
*/
- if (rq && rq->q == hctx->queue && rq->mq_hctx == hctx)
- return iter_data->fn(hctx, rq, iter_data->data, reserved);
- return true;
+ rq = blk_mq_find_and_get_req(tags, bitnr);
+ if (!rq)
+ return true;
+
+ if (rq->q == hctx->queue && rq->mq_hctx == hctx)
+ ret = iter_data->fn(hctx, rq, iter_data->data, reserved);
+ blk_mq_put_rq_ref(rq);
+ return ret;
}

/**
@@ -264,6 +278,8 @@ 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 = true;
+ bool iter_static_rqs = !!(iter_data->flags & BT_TAG_ITER_STATIC_RQS);

if (!reserved)
bitnr += tags->nr_reserved_tags;
@@ -272,16 +288,19 @@ 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
- rq = tags->rqs[bitnr];
+ rq = blk_mq_find_and_get_req(tags, bitnr);
if (!rq)
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);
+
+ if (!(iter_data->flags & BT_TAG_ITER_STARTED) ||
+ blk_mq_request_started(rq))
+ ret = iter_data->fn(rq, iter_data->data, reserved);
+ if (!iter_static_rqs)
+ blk_mq_put_rq_ref(rq);
+ return ret;
}

/**
@@ -348,6 +367,9 @@ 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 466676bc2f0b..d053e80fa6d7 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 9ce64bc4a6c8..556368d2c5b6 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