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

[Michal Hocko <mhocko@suse.cz>]

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@gmail.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@vger.kernel.org # v3.19

Reviewed-by: Michal Hocko <mhocko@suse.cz>

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.

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;
-			}
 		}
 	}
 

> ---
>  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
> 

-- 
Michal Hocko
SUSE Labs

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