slub: avoid potential NULL dereference or corruption

show_slab_objects() can trigger NULL dereferences or memory corruption.

Another cpu can change its c->page to NULL or c->node to NUMA_NO_NODE
while we use them.

Use ACCESS_ONCE(c->page) and ACCESS_ONCE(c->node) to make sure this
cannot happen.
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -4444,30 +4444,31 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 
 		for_each_possible_cpu(cpu) {
 			struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab, cpu);
+			int node = ACCESS_ONCE(c->node);
 			struct page *page;
 
-			if (!c || c->node < 0)
+			if (node < 0)
 				continue;
-
-			if (c->page) {
-					if (flags & SO_TOTAL)
-						x = c->page->objects;
+			page = ACCESS_ONCE(c->page);
+			if (page) {
+				if (flags & SO_TOTAL)
+					x = page->objects;
 				else if (flags & SO_OBJECTS)
-					x = c->page->inuse;
+					x = page->inuse;
 				else
 					x = 1;
 
 				total += x;
-				nodes[c->node] += x;
+				nodes[node] += x;
 			}
 			page = c->partial;
 
 			if (page) {
 				x = page->pobjects;
-                                total += x;
-                                nodes[c->node] += x;
+				total += x;
+				nodes[node] += x;
 			}
-			per_cpu[c->node]++;
+			per_cpu[node]++;
 		}
 	}