linux-kernel - Re: [Question] Is there a race window between __anon_vma_prepare() with page_lock_anon_vma

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Date:   Wed, 19 May 2021 16:41:45 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Question] Is there a race window between __anon_vma_prepare()
 with page_lock_anon_vma_read() ?

On 2021/5/14 17:16, Miaohe Lin wrote:
> Hi all,
> I am investigating the rmap code, and I found the below possible race window:
> 
> CPU 1								CPU 2
> -----								-----
> page_lock_anon_vma_read
>   rcu_read_lock
>   /* We assume anon_vam == root_anon_vma in this case. */
>   root_anon_vma = READ_ONCE(anon_vma->root);
>   root_anon_vma is *released* somewhere unfortunately.
>   down_read_trylock(&root_anon_vma->rwsem)
> 								__anon_vma_prepare
> 								  anon_vma_alloc
> 								    root_anon_vma is *allocated* here.
> 								    init_rwsem(&anon_vma->rwsem);
>   !page_mapped(page)
>     up_read(&root_anon_vma->rwsem); -- *Oops!*
> 
> root_anon_vma->rwsem is reinitialized after locked. And reinitialized anon_vma->rwsem will be
> unlocked without lock first.
> 
> I think this could happen due to the subtle SLAB_TYPESAFE_BY_RCU. But only can occur when anon_vma
> is root anon_vma or they won't operate on the same rwsem.
> Is this will really happen or Am I miss something ? Any reply would be very grateful.
> Many Thanks! :)

Any reply would be very grateful.
Many thanks!

>