linux-kernel - [PATCH] slab: Fix nodeid bounds check for non-contiguous node IDs

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Date:	Mon, 1 Dec 2014 09:16:07 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] slab: Fix nodeid bounds check for non-contiguous node IDs

The bounds check for nodeid in ____cache_alloc_node gives false
positives on machines where the node IDs are not contiguous, leading
to a panic at boot time.  For example, on a POWER8 machine the node
IDs are typically 0, 1, 16 and 17.  This means that num_online_nodes()
returns 4, so when ____cache_alloc_node is called with nodeid = 16 the
VM_BUG_ON triggers.

To fix this, we instead compare the nodeid with MAX_NUMNODES, and
additionally make sure it isn't negative (since nodeid is an int).
The check is there mainly to protect the array dereference in the
get_node() call in the next line, and the array being dereferenced is
of size MAX_NUMNODES.  If the nodeid is in range but invalid, the
BUG_ON in the next line will catch that.

Signed-off-by: Paul Mackerras <paulus@...ba.org>
---
diff --git a/mm/slab.c b/mm/slab.c
index eb2b2ea..f34e053 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3076,7 +3076,7 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
 	void *obj;
 	int x;

-	VM_BUG_ON(nodeid > num_online_nodes());
+	VM_BUG_ON(nodeid < 0 || nodeid >= MAX_NUMNODES);
 	n = get_node(cachep, nodeid);
 	BUG_ON(!n);

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