usercopy: Allow strict enforcement of whitelists

This introduces CONFIG_HARDENED_USERCOPY_FALLBACK to control the
behavior of hardened usercopy whitelist violations. By default, whitelist
violations will continue to WARN() so that any bad or missing usercopy
whitelists can be discovered without being too disruptive.

If this config is disabled at build time or a system is booted with
"slab_common.usercopy_fallback=0", usercopy whitelists will BUG() instead
of WARN(). This is useful for admins that want to use usercopy whitelists
immediately.

Suggested-by: Matthew Garrett <mjg59@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/mm/slub.c b/mm/slub.c
index 6d9b1e7..862d835 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3859,7 +3859,8 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
 	 * whitelists.
 	 */
 	object_size = slab_ksize(s);
-	if (offset <= object_size && n <= object_size - offset) {
+	if (usercopy_fallback &&
+	    offset <= object_size && n <= object_size - offset) {
 		usercopy_warn("SLUB object", s->name, to_user, offset, n);
 		return;
 	}