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

[Leon Yu]

On Tue, Mar 3, 2015 at 9:36 PM, Michal Hocko <mhocko@xxxxxxx> wrote:
> On Tue 03-03-15 20:02:15, Leon Yu 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
>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
>
> I think we can safely remove the following code as well, because it is
> anon_vma_clone which is responsible to do all the cleanups.

Thanks for the input, I'll send v2 with your cleanup.

- Leon

> diff --git a/mm/mmap.c b/mm/mmap.c
> index 943c6ad18b1d..06a6076c92e5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -774,10 +774,8 @@ again:                     remove_next = 1 + (end > next->vm_end);
>
>                         importer->anon_vma = exporter->anon_vma;
>                         error = anon_vma_clone(importer, exporter);
> -                       if (error) {
> -                               importer->anon_vma = NULL;
> +                       if (error)
>                                 return error;
> -                       }
>                 }
>         }
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