mm/kfence: randomize the freelist on initialization

commit 870ff19251bf3910dda7a7245da826924045fedd upstream.

Randomize the KFENCE freelist during pool initialization to make
allocation patterns less predictable.  This is achieved by shuffling the
order in which metadata objects are added to the freelist using
get_random_u32_below().

Additionally, ensure the error path correctly calculates the address range
to be reset if initialization fails, as the address increment logic has
been moved to a separate loop.

Link: https://lkml.kernel.org/r/20260120161510.3289089-1-pimyn@google.com
Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
Signed-off-by: Pimyn Girgis <pimyn@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Ernesto Martnez Garca <ernesto.martinezgarcia@tugraz.at>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Pimyn Girgis <pimyn@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 21 insertions, 4 deletions

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index c49bc76b3a38..e1a555eeec45 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -520,7 +520,7 @@ static bool __init kfence_init_pool(void)
 {
 	unsigned long addr = (unsigned long)__kfence_pool;
 	struct page *pages;
-	int i;
+	int i, rand;
 	char *p;
 
 	if (!__kfence_pool)
@@ -576,13 +576,30 @@ static bool __init kfence_init_pool(void)
 		INIT_LIST_HEAD(&meta->list);
 		raw_spin_lock_init(&meta->lock);
 		meta->state = KFENCE_OBJECT_UNUSED;
-		meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
-		list_add_tail(&meta->list, &kfence_freelist);
+		/* Use addr to randomize the freelist. */
+		meta->addr = i;
 
 		/* Protect the right redzone. */
-		if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
+		if (unlikely(!kfence_protect(addr + 2 * i * PAGE_SIZE + PAGE_SIZE))) {
+			addr += 2 * i * PAGE_SIZE;
 			goto err;
+		}
+	}
+
+	for (i = CONFIG_KFENCE_NUM_OBJECTS; i > 0; i--) {
+		rand = get_random_u32() % i;
+		swap(kfence_metadata[i - 1].addr, kfence_metadata[rand].addr);
+	}
 
+	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
+		struct kfence_metadata *meta_1 = &kfence_metadata[i];
+		struct kfence_metadata *meta_2 = &kfence_metadata[meta_1->addr];
+
+		list_add_tail(&meta_2->list, &kfence_freelist);
+	}
+
+	for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
+		kfence_metadata[i].addr = addr;
 		addr += 2 * PAGE_SIZE;
 	}