Re: [RFC] block: fix access of uninitialized pointer address in bt_for_each()

["yukuai (C)" <yukuai3@huawei.com>]

on 2020/4/17 22:26, Bart Van Assche wrote:

> The alloc/free info refers to a data structure owned by the pipe
> implementation. The use-after-free report refers to a data structure
> owned by the block layer. How can that report make sense?

Indeed, I'm comfused here, too.

>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 7ed16ed13976..48b74d0085c7 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -485,6 +485,7 @@ static void __blk_mq_free_request(struct request *rq)
>>   	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
>>   	const int sched_tag = rq->internal_tag;
>>   
>> +	hctx->tags->rqs[rq->tag] = NULL;
>>   	if (rq->tag != -1)
>>   		blk_mq_put_tag(hctx, hctx->tags, ctx, rq->tag);
>>   	if (sched_tag != -1)
> 
> Can the above change trigger the following assignment?
> 
> hctx->tags->rqs[-1] = NULL?

My bad, should be inside 'if'.

> static inline void *kcalloc_node(size_t n, size_t size, gfp_t flags,
>                                   int node)
> {
> 	return kmalloc_array_node(n, size, flags | __GFP_ZERO, node);
> }
> 
> I think this means that kcalloc_node() already zeroes the allocated
> memory and hence that changing kcalloc() into kzalloc() is not necessary.

You are right.

>> @@ -196,6 +196,7 @@ static inline void blk_mq_put_driver_tag_hctx(struct blk_mq_hw_ctx *hctx,
>>   	if (rq->tag == -1 || rq->internal_tag == -1)
>>   		return;
>>   
>> +	hctx->tags->rqs[rq->tag] = NULL;
>>   	__blk_mq_put_driver_tag(hctx, rq);
>>   }
>>   
>> @@ -207,6 +208,7 @@ static inline void blk_mq_put_driver_tag(struct request *rq)
>>   		return;
>>   
>>   	hctx = blk_mq_map_queue(rq->q, rq->mq_ctx->cpu);
>> +	hctx->tags->rqs[rq->tag] = NULL;
>>   	__blk_mq_put_driver_tag(hctx, rq);
>>   }
> 
> I don't think the above changes are sufficient to fix the
> use-after-free. Has it been considered to free the memory that backs
> tags->bitmap_tags only after an RCU grace period has expired? See also
> blk_mq_free_tags().

As you pointed out, kcalloc_node() already zeroes out the memory. What I 
don't understand is that how could 'slab-out-of-bounds in bt_for_each' 
triggered instead UAF.

Thanks!
Yu Kuai