mm: SLAB hardened usercopy support

Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
SLAB allocator to catch any copies that may span objects.

Based on code from PaX and grsecurity.

Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>

1 files changed, 30 insertions, 0 deletions

diff --git a/mm/slab.c b/mm/slab.c
index cc8bbc1e6bc9..5e2d5f349aca 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4477,6 +4477,36 @@ static int __init slab_proc_init(void)
 module_init(slab_proc_init);
 #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 *cachep;
+        unsigned int objnr;
+        unsigned long offset;
+
+        /* Find and validate object. */
+        cachep = page->slab_cache;
+        objnr = obj_to_index(cachep, page, (void *)ptr);
+        BUG_ON(objnr >= cachep->num);
+
+        /* Find offset within object. */
+        offset = ptr - index_to_obj(cachep, page, objnr) - obj_offset(cachep);
+
+        /* Allow address range falling entirely within object size. */
+        if (offset <= cachep->object_size && n <= cachep->object_size - offset)
+                return NULL;
+
+        return cachep->name;
+}
+#endif /* CONFIG_HARDENED_USERCOPY */
+
 /**
  * ksize - get the actual amount of memory allocated for a given object
  * @objp: Pointer to the object