| Message ID | 20210425085753.2617424-8-ming.lei@redhat.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | blk-mq: fix request UAF related with iterating over tagset requests | expand |
On 4/25/21 1:57 AM, Ming Lei wrote: > However, still one request UAF not covered: refcount_inc_not_zero() may > read one freed request, and it will be handled in next patch. This means that patch "blk-mq: clear stale request in tags->rq[] before freeing one request pool" should come before this patch. > @@ -276,12 +277,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data) > rq = tags->static_rqs[bitnr]; > else > rq = tags->rqs[bitnr]; > - if (!rq) > + 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); > + blk_mq_put_rq_ref(rq); > + return ret; > } Even if patches 7/8 and 8/8 would be reordered, the above code introduces a new use-after-free, a use-after-free that is much worse than the UAF in kernel v5.11. The following sequence can be triggered by the above code: * bt_tags_iter() reads tags->rqs[bitnr] and stores the request pointer in the 'rq' variable. * Request 'rq' completes, tags->rqs[bitnr] is cleared and the memory that backs that request is freed. * The memory that backs 'rq' is used for another purpose and the request reference count becomes nonzero. * bt_tags_iter() increments the request reference count and thereby corrupts memory. Bart.
On Sun, Apr 25, 2021 at 11:55:22AM -0700, Bart Van Assche wrote: > On 4/25/21 1:57 AM, Ming Lei wrote: > > However, still one request UAF not covered: refcount_inc_not_zero() may > > read one freed request, and it will be handled in next patch. > > This means that patch "blk-mq: clear stale request in tags->rq[] before > freeing one request pool" should come before this patch. It doesn't matter. This patch only can't avoid the UAF too, we need to grab req->ref to prevent queue from being frozen. > > > @@ -276,12 +277,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data) > > rq = tags->static_rqs[bitnr]; > > else > > rq = tags->rqs[bitnr]; > > - if (!rq) > > + 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); > > + blk_mq_put_rq_ref(rq); > > + return ret; > > } > > Even if patches 7/8 and 8/8 would be reordered, the above code > introduces a new use-after-free, a use-after-free that is much worse > than the UAF in kernel v5.11. The following sequence can be triggered by > the above code: > * bt_tags_iter() reads tags->rqs[bitnr] and stores the request pointer > in the 'rq' variable. > * Request 'rq' completes, tags->rqs[bitnr] is cleared and the memory > that backs that request is freed. > * The memory that backs 'rq' is used for another purpose and the request > reference count becomes nonzero. That means the 'rq' is re-allocated, and it becomes in-flight again. > * bt_tags_iter() increments the request reference count and thereby > corrupts memory. No, When refcount_inc_not_zero() succeeds in bt_tags_iter(), no one can free the request any more until ->fn() returns, why do you think memory corrupts? This pattern isn't different with timeout's usage, is it? If IO activity is allowed during iterating tagset requests, ->fn() and in-flight IO can always be run concurrently. That is caller's responsibility to handle the race. That is why you can see lots callers do quiesce queues before calling blk_mq_tagset_busy_iter(), but quiesce isn't required if ->fn() just READs request only. Your patch or current in-tree code has same 'problem' too, if you think it is a problem. Clearing ->rq[tag] or holding a lock before calling ->fn() can not avoid such thing, can it? Finally it is a request walking in tagset wide, so it should be safe for ->fn to iterate over request in this way. The thing is just that req->tag may become not same with 'bitnr' any more. We can handle it simply by checking if 'req->tag == bitnr' in bt_tags_iter() after the req->ref is grabbed, still not sure if it is absolutely necessary. Thanks, Ming
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 2a37731e8244..489d2db89856 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -264,6 +264,7 @@ 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; if (!reserved) bitnr += tags->nr_reserved_tags; @@ -276,12 +277,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data) rq = tags->static_rqs[bitnr]; else rq = tags->rqs[bitnr]; - if (!rq) + 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); + blk_mq_put_rq_ref(rq); + return ret; } /** @@ -348,6 +352,10 @@ 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. So far we don't support to pass the request reference to + * one new conetxt in @fn. */ 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 e3d1067b10c3..9a4d520740a1 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -925,6 +925,14 @@ 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) { @@ -958,11 +966,7 @@ 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
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 | 14 +++++++++++--- block/blk-mq.c | 14 +++++++++----- block/blk-mq.h | 1 + 3 files changed, 21 insertions(+), 8 deletions(-)