Re: [PATCH v3 7/9] mm/page_owner: avoid null pointer dereference

[Vlastimil Babka <vbabka@suse.cz>]

On 06/17/2016 09:57 AM, js1304@gmail.com wrote:
> From: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
>
> We have dereferenced page_ext before checking it. Lets check it first
> and then used it.
>
> Link: http://lkml.kernel.org/r/1465249059-7883-1-git-send-email-sudipm.mukherjee@gmail.com
> Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Hmm, this is already in mmotm as 
http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-page_owner-use-stackdepot-to-store-stacktrace-fix.patch

But imho it's fixing a problem not related to your patch, but something that the 
commit f86e4271978b missed. So it should separately go to 4.7 ASAP.

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Fixes: f86e4271978b ("mm: check the return value of lookup_page_ext for all call 
sites")


> ---
>  mm/page_owner.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index dc92241..ec6dc18 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -295,13 +295,15 @@ void __dump_page_owner(struct page *page)
>  		.skip = 0
>  	};
>  	depot_stack_handle_t handle;
> -	gfp_t gfp_mask = page_ext->gfp_mask;
> -	int mt = gfpflags_to_migratetype(gfp_mask);
> +	gfp_t gfp_mask;
> +	int mt;
>
>  	if (unlikely(!page_ext)) {
>  		pr_alert("There is not page extension available.\n");
>  		return;
>  	}
> +	gfp_mask = page_ext->gfp_mask;
> +	mt = gfpflags_to_migratetype(gfp_mask);
>
>  	if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags)) {
>  		pr_alert("page_owner info is not active (free page?)\n");
>

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