linux-kernel - [PATCH] mm/page_owner: avoid null pointer dereference

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Date:	Mon,  6 Jun 2016 22:37:39 +0100
From:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: [PATCH] mm/page_owner: avoid null pointer dereference

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

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
---
 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 587dcca..8fa5083 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -291,13 +291,15 @@ 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");
-- 
1.9.1