[PATCH v4 17/35] mm: kfence: Disable KMSAN when checking the canary

Ilya Leoshkevich
2024-06-13T15:40:12Z

KMSAN warns about check_canary() accessing the canary.

The reason is that, even though set_canary() is properly instrumented
and sets shadow, slub explicitly poisons the canary's address range
afterwards.

Unpoisoning the canary is not the right thing to do: only
check_canary() is supposed to ever touch it. Instead, disable KMSAN
checks around canary read accesses.

Reviewed-by: Alexander Potapenko <glider@google.com>
Tested-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
mm/kfence/core.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 964b8482275b..cce330d5b797 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -305,8 +305,14 @@ metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state nex
WRITE_ONCE(meta->state, next);
}

+#ifdef CONFIG_KMSAN
+#define CHECK_CANARY_ATTRIBUTES noinline __no_kmsan_checks
+#else
+#define CHECK_CANARY_ATTRIBUTES inline
+#endif
+
/* Check canary byte at @addr. */
-static inline bool check_canary_byte(u8 *addr)
+static CHECK_CANARY_ATTRIBUTES bool check_canary_byte(u8 *addr)
{
struct kfence_metadata *meta;
unsigned long flags;
@@ -341,7 +347,8 @@ static inline void set_canary(const struct kfence_metadata *meta)
*((u64 *)addr) = KFENCE_CANARY_PATTERN_U64;
}

-static inline void check_canary(const struct kfence_metadata *meta)
+static CHECK_CANARY_ATTRIBUTES void
+check_canary(const struct kfence_metadata *meta)
{
const unsigned long pageaddr = ALIGN_DOWN(meta->addr, PAGE_SIZE);
unsigned long addr = pageaddr;
--
2.45.1