Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[Konstantin Khlebnikov]

On Tue, Mar 3, 2015 at 3:02 PM, Leon Yu <chianglungyu@xxxxxxxxx> wrote:
> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
> to 3.19 and had no luck with 4.0-rc1 neither.
>
> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
> prevent endless growth of anon_vma hierarchy"), I found chances are that
> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
> and eventually underflow when exiting as a result of another call to
> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
> for me.
>
> This patch fixes the underflow by dropping dst->anon_vma when allocation
> fails. It's safe to do so regardless of original value of dst->anon_vma
> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
> Besides, callers don't care dst->anon_vma in such case neither.
>
> Signed-off-by: Leon Yu <chianglungyu@xxxxxxxxx>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@xxxxxxxxxxxxxxx # v3.19

Good catch.

Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx>

That thing already backported into various stable branches so this's
not only for v3.19.

As I see other error paths are fine.

> ---
>  mm/rmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5e3e090..bed3cf2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>         return 0;
>
>   enomem_failure:
> +       /*
> +        * dst->anon_vma is dropped here otherwise its degree can be incorrectly
> +        * decremented in unlink_anon_vmas().
> +        * We can safely do this because calllers of anon_vma_clone() wouldn't
> +        * care dst->anon_vma if anon_vma_clone() failed.
> +        */
> +       dst->anon_vma = NULL;
>         unlink_anon_vmas(dst);
>         return -ENOMEM;
>  }
> --
> 2.3.1
>
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