kfence: zero guard page after out-of-bounds access

After an out-of-bounds accesses, zero the guard page before re-protecting
in kfence_guarded_free().  On one hand this helps make the failure mode of
subsequent out-of-bounds accesses more deterministic, but could also
prevent certain information leaks.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Marco Elver <[email protected]>
Acked-by: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/kfence/core.c

@@ -372,6 +372,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
 
 	/* Restore page protection if there was an OOB access. */
 	if (meta->unprotected_page) {
+		memzero_explicit((void *)ALIGN_DOWN(meta->unprotected_page, PAGE_SIZE), PAGE_SIZE);
 		kfence_protect(meta->unprotected_page);
 		meta->unprotected_page = 0;
 	}