[PATCH] slab: verify pointers before free

Passing an invalid pointer to kfree() and kmem_cache_free() is likely to
cause bad memory corruption or even take down the whole system because the
bad pointer is likely reused immediately due to the per-CPU caches.  Until
now, we don't do any verification for this if CONFIG_DEBUG_SLAB is
disabled.

As suggested by Linus, add PageSlab check to page_to_cache() and
page_to_slab() to verify pointers passed to kfree().  Also, move the
stronger check from cache_free_debugcheck() to kmem_cache_free() to ensure
the passed pointer actually belongs to the cache we're about to free the
object.

For page_to_cache() and page_to_slab(), the assertions should have
virtually no extra cost (two instructions, no data cache pressure) and for
kmem_cache_free() the overhead should be minimal.
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Cc: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/slab.c

@@ -592,6 +592,7 @@ static inline struct kmem_cache *page_get_cache(struct page *page)
 {
 	if (unlikely(PageCompound(page)))
 		page = (struct page *)page_private(page);
+	BUG_ON(!PageSlab(page));
 	return (struct kmem_cache *)page->lru.next;
 }
 
@@ -604,6 +605,7 @@ static inline struct slab *page_get_slab(struct page *page)
 {
 	if (unlikely(PageCompound(page)))
 		page = (struct page *)page_private(page);
+	BUG_ON(!PageSlab(page));
 	return (struct slab *)page->lru.prev;
 }
 
@@ -2669,15 +2671,6 @@ static void *cache_free_debugcheck(struct kmem_cache *cachep, void *objp,
 	kfree_debugcheck(objp);
 	page = virt_to_page(objp);
 
-	if (page_get_cache(page) != cachep) {
-		printk(KERN_ERR "mismatch in kmem_cache_free: expected "
-				"cache %p, got %p\n",
-		       page_get_cache(page), cachep);
-		printk(KERN_ERR "%p is %s.\n", cachep, cachep->name);
-		printk(KERN_ERR "%p is %s.\n", page_get_cache(page),
-		       page_get_cache(page)->name);
-		WARN_ON(1);
-	}
 	slabp = page_get_slab(page);
 
 	if (cachep->flags & SLAB_RED_ZONE) {
@@ -3393,6 +3386,8 @@ void kmem_cache_free(struct kmem_cache *cachep, void *objp)
 {
 	unsigned long flags;
 
+	BUG_ON(virt_to_cache(objp) != cachep);
+
 	local_irq_save(flags);
 	__cache_free(cachep, objp);
 	local_irq_restore(flags);