hardening: Enable KFENCE in the hardening config

KFENCE is not a security mitigation mechanism (due to sampling), but has
the performance characteristics of unintrusive hardening techniques.
When used at scale, however, it improves overall security by allowing
kernel developers to detect heap memory-safety bugs cheaply.

Link: https://lkml.kernel.org/r/79B9A832-B3DE-4229-9D87-748B2CFB7D12@kernel.org
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/r/20240212130116.997627-1-elver@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/configs/hardening.config

@@ -45,6 +45,9 @@ CONFIG_UBSAN_BOUNDS=y
 # CONFIG_UBSAN_ENUM
 # CONFIG_UBSAN_ALIGNMENT
 
+# Sampling-based heap out-of-bounds and use-after-free detection.
+CONFIG_KFENCE=y
+
 # Linked list integrity checking.
 CONFIG_LIST_HARDENED=y