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. The usage of
memset_no_sanitize_memory() does not degrade the generated code
quality.
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@xxxxxxxxxxxxx>
---
mm/kmsan/hooks.c | 2 +-
mm/slub.c | 10 ++++++----
2 files changed, 7 insertions(+), 5 deletions(-)
diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 7b5814412e9f..7a30274b893c 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -76,7 +76,7 @@ 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 63d281dfacdb..169e5f645ea8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1030,7 +1030,8 @@ 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);
+ memset_no_sanitize_memory(p - s->red_left_pad, val,
+ s->red_left_pad);
if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
/*
@@ -1043,12 +1044,13 @@ 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_no_sanitize_memory(p, POISON_FREE, poison_size - 1);
+ memset_no_sanitize_memory(p + poison_size - 1, POISON_END, 1);
}
if (s->flags & SLAB_RED_ZONE)
- memset(p + poison_size, val, s->inuse - poison_size);
+ memset_no_sanitize_memory(p + poison_size, val,
+ s->inuse - poison_size);
}
static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
--
2.41.0
