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

From: Tejun Heo
Date: Mon Sep 13 2021 - 13:50:47 EST


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.
> 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.