mm: SLUB hardened usercopy support

Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
SLUB allocator to catch any copies that may span objects. Includes a
redzone handling fix discovered by Michael Ellerman.

Based on code from PaX and grsecurity.
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Reviwed-by: Laura Abbott <labbott@redhat.com>

init/Kconfig

@@ -1766,6 +1766,7 @@ config SLAB
 
 config SLUB
 	bool "SLUB (Unqueued Allocator)"
+	select HAVE_HARDENED_USERCOPY_ALLOCATOR
 	help
 	   SLUB is a slab allocator that minimizes cache line usage
 	   instead of managing queues of cached objects (SLAB approach).

mm/slub.c

@@ -3614,6 +3614,46 @@ void *__kmalloc_node(size_t size, gfp_t flags, int node)
 EXPORT_SYMBOL(__kmalloc_node);
 #endif
 
+#ifdef CONFIG_HARDENED_USERCOPY
+/*
+ * Rejects objects that are incorrectly sized.
+ *
+ * Returns NULL if check passes, otherwise const char * to name of cache
+ * to indicate an error.
+ */
+const char *__check_heap_object(const void *ptr, unsigned long n,
+				struct page *page)
+{
+	struct kmem_cache *s;
+	unsigned long offset;
+	size_t object_size;
+
+	/* Find object and usable object size. */
+	s = page->slab_cache;
+	object_size = slab_ksize(s);
+
+	/* Reject impossible pointers. */
+	if (ptr < page_address(page))
+		return s->name;
+
+	/* Find offset within object. */
+	offset = (ptr - page_address(page)) % s->size;
+
+	/* Adjust for redzone and reject if within the redzone. */
+	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
+		if (offset < s->red_left_pad)
+			return s->name;
+		offset -= s->red_left_pad;
+	}
+
+	/* Allow address range falling entirely within object size. */
+	if (offset <= object_size && n <= object_size - offset)
+		return NULL;
+
+	return s->name;
+}
+#endif /* CONFIG_HARDENED_USERCOPY */
+
 static size_t __ksize(const void *object)
 {
 	struct page *page;