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

[yukuai (C)]

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