linux-kernel - Re: [PATCH v4 3/3] mm: fix use-after-free when anon vma name is used after vma is freed

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Date:   Tue, 22 Feb 2022 09:06:31 +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,
        hannes@...xchg.org, ebiederm@...ssion.com, brauner@...nel.org,
        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,
        kernel-team@...roid.com,
        syzbot+aa7b3d4b35f9dc46a366@...kaller.appspotmail.com
Subject: Re: [PATCH v4 3/3] mm: fix use-after-free when anon vma name is used
 after vma is freed

On Mon 21-02-22 21:40:25, 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

This seems to be stale because bare const char pointer is not passed in
the call chain. In fact I am not even sure there is any actual UAF here
after the rework.
Could you be more specific in describing the scenario?

> Fix this by raising the name refcount and stabilizing it.
> 
> Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Reported-by: syzbot+aa7b3d4b35f9dc46a366@...kaller.appspotmail.com
> ---
> changes in v3:
> - Reapplied the fix after code refactoring, per Michal Hocko
> 
>  mm/madvise.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index a395884aeecb..00e8105430e9 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -140,6 +140,8 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
>  /*
>   * Update the vm_flags on region of a vma, splitting it or merging it as
>   * necessary.  Must be called with mmap_sem held for writing;
> + * Caller should ensure anon_name stability by raising its refcount even when
> + * anon_name belongs to a valid vma because this function might free that vma.
>   */
>  static int madvise_update_vma(struct vm_area_struct *vma,
>  			      struct vm_area_struct **prev, unsigned long start,
> @@ -1021,8 +1023,10 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>  	}
>  
>  	anon_name = vma_anon_name(vma);
> +	anon_vma_name_get(anon_name);
>  	error = madvise_update_vma(vma, prev, start, end, new_flags,
>  				   anon_name);
> +	anon_vma_name_put(anon_name);
>  
>  out:
>  	/*
> -- 
> 2.35.1.473.g83b2b277ed-goog

-- 
Michal Hocko
SUSE Labs