[PATCH v6 6/9] kfence, kasan: make KFENCE compatible with KASAN

Marco Elver
2020-10-29T13:17:23Z

From: Alexander Potapenko <glider@google.com>

We make KFENCE compatible with KASAN for testing KFENCE itself. In
particular, KASAN helps to catch any potential corruptions to KFENCE
state, or other corruptions that may be a result of freepointer
corruptions in the main allocators.

To indicate that the combination of the two is generally discouraged,
CONFIG_EXPERT=y should be set. It also gives us the nice property that
KFENCE will be build-tested by allyesconfig builds.

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Co-developed-by: Marco Elver <elver@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
---
v5:
* Also guard kasan_unpoison_shadow with is_kfence_address(), as it may
be called from SL*B internals, currently ksize().
* Make kasan_record_aux_stack() compatible with KFENCE, which may be
called from outside KASAN runtime.
---
lib/Kconfig.kfence | 2 +-
mm/kasan/common.c | 15 +++++++++++++++
mm/kasan/generic.c | 3 ++-
3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
index d24baa3bce4a..639b48cc75d4 100644
--- a/lib/Kconfig.kfence
+++ b/lib/Kconfig.kfence
@@ -5,7 +5,7 @@ config HAVE_ARCH_KFENCE

menuconfig KFENCE
bool "KFENCE: low-overhead sampling-based memory safety error detector"
- depends on HAVE_ARCH_KFENCE && !KASAN && (SLAB || SLUB)
+ depends on HAVE_ARCH_KFENCE && (!KASAN || EXPERT) && (SLAB || SLUB)
depends on JUMP_LABEL # To ensure performance, require jump labels
select STACKTRACE
help
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 950fd372a07e..ac1d404fb41e 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -18,6 +18,7 @@
#include <linux/init.h>
#include <linux/kasan.h>
#include <linux/kernel.h>
+#include <linux/kfence.h>
#include <linux/kmemleak.h>
#include <linux/linkage.h>
#include <linux/memblock.h>
@@ -141,6 +142,14 @@ void kasan_unpoison_shadow(const void *address, size_t size)
*/
address = reset_tag(address);

+ /*
+ * We may be called from SL*B internals, such as ksize(): with a size
+ * not a multiple of machine-word size, avoid poisoning the invalid
+ * portion of the word for KFENCE memory.
+ */
+ if (is_kfence_address(address))
+ return;
+
kasan_poison_shadow(address, size, tag);

if (size & KASAN_SHADOW_MASK) {
@@ -396,6 +405,9 @@ static bool __kasan_slab_free(struct kmem_cache *cache, void *object,
tagged_object = object;
object = reset_tag(object);

+ if (is_kfence_address(object))
+ return false;
+
if (unlikely(nearest_obj(cache, virt_to_head_page(object), object) !=
object)) {
kasan_report_invalid_free(tagged_object, ip);
@@ -444,6 +456,9 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
if (unlikely(object == NULL))
return NULL;

+ if (is_kfence_address(object))
+ return (void *)object;
+
redzone_start = round_up((unsigned long)(object + size),
KASAN_SHADOW_SCALE_SIZE);
redzone_end = round_up((unsigned long)object + cache->object_size,
diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index 248264b9cb76..1069ecd1cd55 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -21,6 +21,7 @@
#include <linux/init.h>
#include <linux/kasan.h>
#include <linux/kernel.h>
+#include <linux/kfence.h>
#include <linux/kmemleak.h>
#include <linux/linkage.h>
#include <linux/memblock.h>
@@ -332,7 +333,7 @@ void kasan_record_aux_stack(void *addr)
struct kasan_alloc_meta *alloc_info;
void *object;

- if (!(page && PageSlab(page)))
+ if (is_kfence_address(addr) || !(page && PageSlab(page)))
return;

cache = page->slab_cache;
--
2.29.1.341.ge80a0c044ae-goog