Re: [PATCH] mm: fix null pointer dereference in wait_iff_congested()

[Minchan Kim <minchan@kernel.org>]

Hello Zlatko,

On Fri, Dec 28, 2012 at 03:16:38AM +0100, Zlatko Calusic wrote:
> From: Zlatko Calusic <zlatko.calusic@iskon.hr>
> 
> The unintended consequence of commit 4ae0a48b is that
> wait_iff_congested() can now be called with NULL struct zone*
> producing kernel oops like this:

For good description, it would be better to write simple pseudo code
flow to show how NULL-zone pass into wait_iff_congested because
kswapd code flow is too complex.

As I see the code, we have following line above wait_iff_congested.

if (!unbalanced_zone || blah blah)
        break;

How can NULL unbalanced_zone reach wait_iff_congested?

> 
> BUG: unable to handle kernel NULL pointer dereference
> IP: [<ffffffff811542d9>] wait_iff_congested+0x59/0x140
> 
> This trivial patch fixes it.
> 
> Reported-by: Zhouping Liu <zliu@redhat.com>
> Reported-and-tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Hugh Dickins <hughd@google.com>
> Signed-off-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 02bcfa3..e55ce55 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2782,7 +2782,7 @@ loop_again:
>  		if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {
>  			if (has_under_min_watermark_zone)
>  				count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
> -			else
> +			else if (unbalanced_zone)
>  				wait_iff_congested(unbalanced_zone, BLK_RW_ASYNC, HZ/10);
>  		}
>  
> -- 
> 1.8.1.rc3
> 
> -- 
> Zlatko
> 
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-- 
Kind regards,
Minchan Kim

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