Re: [PATCH] mm: pagewalk: prevent positive return value of walk_page_test() from being passed to callers (Re: [PATCH] mm: fix do_mbind return value)

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Thu, Mar 05, 2015 at 08:02:27AM +0000, Horiguchi Naoya(堀口 直也) wrote:
...
> ---
> From 107fa3fb256bddff40a882c90af717af9863aed7 Mon Sep 17 00:00:00 2001
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Date: Thu, 5 Mar 2015 16:37:37 +0900
> Subject: [PATCH] mm: pagewalk: prevent positive return value of
>  walk_page_test() from being passed to callers
> 
> walk_page_test() is purely pagewalk's internal stuff, and its positive return
> values are not intended to be passed to the callers of pagewalk. However, in
> the current code if the last vma in the do-while loop in walk_page_range()
> happens to return a positive value, it leaks outside walk_page_range().
> So the user visible effect is invalid/unexpected return value (according to
> the reporter, mbind() causes it.)
> 
> This patch fixes it simply by reinitializing the return value after checked.
> 
> Another exposed interface, walk_page_vma(), already returns 0 for such cases
> so no problem.
> 
> Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")

This is not a right tag. To be precise, the bug was introduced by commit
fafaa4264eba ("pagewalk: improve vma handling"), so

  Fixes fafaa4264eba ("pagewalk: improve vma handling")

is right.

Thanks,
Naoya Horiguchi

> Reported-by: Kazutomo Yoshii <kazutomo.yoshii@gmail.com>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  mm/pagewalk.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 75c1f2878519..29f2f8b853ae 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -265,8 +265,15 @@ int walk_page_range(unsigned long start, unsigned long end,
>  			vma = vma->vm_next;
>  
>  			err = walk_page_test(start, next, walk);
> -			if (err > 0)
> +			if (err > 0) {
> +				/*
> +				 * positive return values are purely for
> +				 * controlling the pagewalk, so should never
> +				 * be passed to the callers.
> +				 */
> +				err = 0;
>  				continue;
> +			}
>  			if (err < 0)
>  				break;
>  		}
> -- 
> 1.9.3
>