Re: [PATCH v2 14/33] kmsan: Support SLAB_POISON

[Alexander Potapenko]

On Tue, Nov 21, 2023 at 11:02 PM Ilya Leoshkevich  wrote:
>
> 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 
> ---
>  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,

As I wrote in patch 13/33, let's try to use __memset() here (with a
comment that we want to preserve the previously poisoned memory)

[PATCH v2 14/33] kmsan: Support SLAB_POISON

[Ilya Leoshkevich]

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 
---
 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