[PATCH v4 12/35] kmsan: Support SLAB_POISON

Ilya Leoshkevich
2024-06-13T15:40:11Z

Avoid false KMSAN negatives with SLUB_DEBUG by allowing
kmsan_slab_free() to poison the freed memory, and by preventing
init_object() from unpoisoning new allocations by using __memset().

There are two alternatives to this approach. First, init_object()
can be marked with __no_sanitize_memory. This annotation should be used
with great care, because it drops all instrumentation from the
function, and any shadow writes will be lost. Even though this is not a
concern with the current init_object() implementation, this may change
in the future.

Second, kmsan_poison_memory() calls may be added after memset() calls.
The downside is that init_object() is called from
free_debug_processing(), in which case poisoning will erase the
distinction between simply uninitialized memory and UAF.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
mm/kmsan/hooks.c | 2 +-
mm/slub.c | 13 +++++++++----
2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 267d0afa2e8b..26d86dfdc819 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -74,7 +74,7 @@ void kmsan_slab_free(struct kmem_cache *s, void *object)
return;

/* RCU slabs could be legally used after free within the RCU period */
- if (unlikely(s->flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)))
+ if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
return;
/*
* If there's a constructor, freed memory must remain in the same state
diff --git a/mm/slub.c b/mm/slub.c
index 1373ac365a46..4dd55cabe701 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1139,7 +1139,12 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
unsigned int poison_size = s->object_size;

if (s->flags & SLAB_RED_ZONE) {
- memset(p - s->red_left_pad, val, s->red_left_pad);
+ /*
+ * Use __memset() here and below in order to avoid overwriting
+ * the KMSAN shadow. Keeping the shadow makes it possible to
+ * distinguish uninit-value from use-after-free.
+ */
+ __memset(p - s->red_left_pad, val, s->red_left_pad);

if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
/*
@@ -1152,12 +1157,12 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
}

if (s->flags & __OBJECT_POISON) {
- memset(p, POISON_FREE, poison_size - 1);
- p[poison_size - 1] = POISON_END;
+ __memset(p, POISON_FREE, poison_size - 1);
+ __memset(p + poison_size - 1, POISON_END, 1);
}

if (s->flags & SLAB_RED_ZONE)
- memset(p + poison_size, val, s->inuse - poison_size);
+ __memset(p + poison_size, val, s->inuse - poison_size);
}

static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
--
2.45.1