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
