linux-kernel - Re: [PATCH v2 1/1] mm: fix use-after-free when anon vma name is used after vma is freed

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Message-ID: <YgUHzSqltDp2dr70@dhcp22.suse.cz>
Date:   Thu, 10 Feb 2022 13:40:45 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, ccross@...gle.com,
        sumit.semwal@...aro.org, dave.hansen@...el.com,
        keescook@...omium.org, willy@...radead.org,
        kirill.shutemov@...ux.intel.com, vbabka@...e.cz,
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        legion@...nel.org, ran.xiaokai@....com.cn, sashal@...nel.org,
        chris.hyser@...cle.com, dave@...olabs.net, pcc@...gle.com,
        caoxiaofeng@...ong.com, david@...hat.com, gorcunov@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
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        syzbot+aa7b3d4b35f9dc46a366@...kaller.appspotmail.com
Subject: Re: [PATCH v2 1/1] mm: fix use-after-free when anon vma name is used
 after vma is freed

On Wed 09-02-22 20:32:15, Suren Baghdasaryan wrote:
> When adjacent vmas are being merged it can result in the vma that was
> originally passed to madvise_update_vma being destroyed. In the current
> implementation, the name parameter passed to madvise_update_vma points
> directly to vma->anon_name->name and it is used after the call to
> vma_merge. In the cases when vma_merge merges the original vma and
> destroys it, this will result in use-after-free bug as shown below:
> 
> madvise_vma_behavior << passes vma->anon_name->name as name param
>   madvise_update_vma(name)
>     vma_merge
>       __vma_adjust
>         vm_area_free <-- frees the vma
>     replace_vma_anon_name(name) <-- UAF
> 
> Fix this by raising the name refcount and stabilizing it. Introduce
> vma_anon_name_{get/put} API for this purpose.

What is the reason that madvise_update_vma uses the naked name rather
than the encapsulated anon_vma_name? This really just begs for problems.
-- 
Michal Hocko
SUSE Labs