Re: [PATCH v2] block, bfq: fix UAF in bfq_io_set_weight_legacy()

[Tejun Heo]

Hello,

On Fri, Sep 10, 2021 at 11:46:42AM +0800, Li Jinlin wrote:
> Freeing bfqg is protected by queue lock in blkcg_deactivate_policy(), 
> but getting/using bfqg is protected by blkcg lock in
> bfq_io_set_weight_legacy(). If bfq_io_set_weight_legacy() get bfqg
> before freeing bfqg and use bfqg in the after, the use-after-free
> will occur.
> 
> CPU0                             CPU1
> blkcg_deactivate_policy
>   spin_lock_irq(&q->queue_lock)
>                                  bfq_io_set_weight_legacy  
>                                    spin_lock_irq(&blkcg->lock)
>                                    blkg_to_bfqg(blkg)
>                                      pd_to_bfqg(blkg->pd[pol->plid])
>                                      ^^^^^^blkg->pd[pol->plid] != NULL
>                                            bfqg != NULL
>   pol->pd_free_fn(blkg->pd[pol->plid])
>     pd_to_bfqg(blkg->pd[pol->plid])
>     bfqg_put(bfqg)
>       kfree(bfqg)
>   blkg->pd[pol->plid] = NULL
>   spin_unlock_irq(q->queue_lock);
>                                    bfq_group_set_weight(bfqg, val, 0)
>                                      bfqg->entity.new_weight
>                                      ^^^^^^trigger uaf here 
>                                    spin_unlock_irq(&blkcg->lock);
> 
> To fix this use-after-free, instead of holding blkcg->lock while
> walking ->blkg_list and getting/using bfqg, RCU walk ->blkg_list and
> hold the blkg's queue lock while getting/using bfqg.

I think this is a bug in blkcg_deactivate_policy() than the other way
around. blkgs are protected by both q and blkcg locks and holding either
should stabilize them. The blkcg lock nests inside q lock, so I think
blkcg_deactivate_policy() just needs to grab the matching blkcg lock before
trying to destroy blkgs.

Thanks.

-- 
tejun