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

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Date:	Fri, 24 Jun 2016 13:19:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	js1304@...il.com, mgorman@...hsingularity.net,
	Minchan Kim <minchan@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Sasha Levin <sasha.levin@...cle.com>,
	Sudip Mukherjee <sudipm.mukherjee@...il.com>,
	Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3 7/9] mm/page_owner: avoid null pointer dereference

On Fri, 17 Jun 2016 15:32:20 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:

> On 06/17/2016 09:57 AM, js1304@...il.com wrote:
> > From: Sudip Mukherjee <sudipm.mukherjee@...il.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@...ethink.co.uk>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....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@...e.cz>
> Fixes: f86e4271978b ("mm: check the return value of lookup_page_ext for all call 
> sites")

Thanks, I reordered Sudip's patch.



From: Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: mm/page_owner: avoid null pointer dereference

We have dereferenced page_ext before checking it. Lets check it first
and then used it.

Fixes: f86e4271978b ("mm: check the return value of lookup_page_ext for all call sites")
Link: http://lkml.kernel.org/r/1465249059-7883-1-git-send-email-sudipm.mukherjee@gmail.com
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 mm/page_owner.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff -puN mm/page_owner.c~mm-page_owner-use-stackdepot-to-store-stacktrace-fix mm/page_owner.c
--- a/mm/page_owner.c~mm-page_owner-use-stackdepot-to-store-stacktrace-fix
+++ a/mm/page_owner.c
@@ -207,13 +207,15 @@ void __dump_page_owner(struct page *page
 		.nr_entries = page_ext->nr_entries,
 		.entries = &page_ext->trace_entries[0],
 	};
-	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");
_