[PATCH 1/2] kasan: add ability to detect double-kmem_cache_destroy()

[PATCH 1/2] kasan: add ability to detect double-kmem_cache_destroy()

[Marco Elver]

Because mm/slab_common.c is not instrumented with software KASAN modes,
it is not possible to detect use-after-free of the kmem_cache passed
into kmem_cache_destroy(). In particular, because of the s->refcount--
and subsequent early return if non-zero, KASAN would never be able to
see the double-free via kmem_cache_free(kmem_cache, s). To be able to
detect a double-kmem_cache_destroy(), check accessibility of the
kmem_cache, and in case of failure return early.

While KASAN_HW_TAGS is able to detect such bugs, by checking
accessibility and returning early we fail more gracefully and also
avoid corrupting reused objects (where tags mismatch).

A recent case of a double-kmem_cache_destroy() was detected by KFENCE:
https://lkml.kernel.org/r/0000000000003f654905c168b09d@google.com
, which was not detectable by software KASAN modes.

Signed-off-by: Marco Elver <elver@google.com>
---
 mm/slab_common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index e5d080a93009..4bef4b6a2c76 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -491,7 +491,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
 {
 	int err;
 
-	if (unlikely(!s))
+	if (unlikely(!s || !kasan_check_byte(s)))
 		return;
 
 	cpus_read_lock();
-- 
2.34.0.rc2.393.gf8c9666880-goog

[PATCH 2/2] kasan: test: add test case for double-kmem_cache_destroy()

[Marco Elver]

Add a test case for double-kmem_cache_destroy() detection.

Signed-off-by: Marco Elver <elver@google.com>
---
 lib/test_kasan.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index 40f7274297c1..4da4b214ed06 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -866,6 +866,16 @@ static void kmem_cache_invalid_free(struct kunit *test)
 	kmem_cache_destroy(cache);
 }
 
+static void kmem_cache_double_destroy(struct kunit *test)
+{
+	struct kmem_cache *cache;
+
+	cache = kmem_cache_create("test_cache", 200, 0, 0, NULL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cache);
+	kmem_cache_destroy(cache);
+	KUNIT_EXPECT_KASAN_FAIL(test, kmem_cache_destroy(cache));
+}
+
 static void kasan_memchr(struct kunit *test)
 {
 	char *ptr;
@@ -1183,6 +1193,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(ksize_uaf),
 	KUNIT_CASE(kmem_cache_double_free),
 	KUNIT_CASE(kmem_cache_invalid_free),
+	KUNIT_CASE(kmem_cache_double_destroy),
 	KUNIT_CASE(kasan_memchr),
 	KUNIT_CASE(kasan_memcmp),
 	KUNIT_CASE(kasan_strings),
-- 
2.34.0.rc2.393.gf8c9666880-goog

Re: [PATCH 1/2] kasan: add ability to detect double-kmem_cache_destroy()

[Vlastimil Babka]

On 11/19/21 15:22, Marco Elver wrote:
> Because mm/slab_common.c is not instrumented with software KASAN modes,
> it is not possible to detect use-after-free of the kmem_cache passed
> into kmem_cache_destroy(). In particular, because of the s->refcount--
> and subsequent early return if non-zero, KASAN would never be able to
> see the double-free via kmem_cache_free(kmem_cache, s). To be able to
> detect a double-kmem_cache_destroy(), check accessibility of the
> kmem_cache, and in case of failure return early.
> 
> While KASAN_HW_TAGS is able to detect such bugs, by checking
> accessibility and returning early we fail more gracefully and also
> avoid corrupting reused objects (where tags mismatch).
> 
> A recent case of a double-kmem_cache_destroy() was detected by KFENCE:
> https://lkml.kernel.org/r/0000000000003f654905c168b09d@google.com
> , which was not detectable by software KASAN modes.
> 
> Signed-off-by: Marco Elver <elver@google.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e5d080a93009..4bef4b6a2c76 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -491,7 +491,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  {
>  	int err;
>  
> -	if (unlikely(!s))
> +	if (unlikely(!s || !kasan_check_byte(s)))
>  		return;
>  
>  	cpus_read_lock();
> 

Re: [PATCH 1/2] kasan: add ability to detect double-kmem_cache_destroy()

[Andrey Konovalov]

On Fri, Nov 19, 2021 at 3:22 PM Marco Elver <elver@google.com> wrote:
>
> Because mm/slab_common.c is not instrumented with software KASAN modes,
> it is not possible to detect use-after-free of the kmem_cache passed
> into kmem_cache_destroy(). In particular, because of the s->refcount--
> and subsequent early return if non-zero, KASAN would never be able to
> see the double-free via kmem_cache_free(kmem_cache, s). To be able to
> detect a double-kmem_cache_destroy(), check accessibility of the
> kmem_cache, and in case of failure return early.
>
> While KASAN_HW_TAGS is able to detect such bugs, by checking
> accessibility and returning early we fail more gracefully and also
> avoid corrupting reused objects (where tags mismatch).
>
> A recent case of a double-kmem_cache_destroy() was detected by KFENCE:
> https://lkml.kernel.org/r/0000000000003f654905c168b09d@google.com
> , which was not detectable by software KASAN modes.
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e5d080a93009..4bef4b6a2c76 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -491,7 +491,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  {
>         int err;
>
> -       if (unlikely(!s))
> +       if (unlikely(!s || !kasan_check_byte(s)))
>                 return;
>
>         cpus_read_lock();
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Thanks!

Re: [PATCH 2/2] kasan: test: add test case for double-kmem_cache_destroy()

[Andrey Konovalov]

On Fri, Nov 19, 2021 at 3:22 PM Marco Elver <elver@google.com> wrote:
>
> Add a test case for double-kmem_cache_destroy() detection.
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  lib/test_kasan.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 40f7274297c1..4da4b214ed06 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -866,6 +866,16 @@ static void kmem_cache_invalid_free(struct kunit *test)
>         kmem_cache_destroy(cache);
>  }
>
> +static void kmem_cache_double_destroy(struct kunit *test)
> +{
> +       struct kmem_cache *cache;
> +
> +       cache = kmem_cache_create("test_cache", 200, 0, 0, NULL);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cache);
> +       kmem_cache_destroy(cache);
> +       KUNIT_EXPECT_KASAN_FAIL(test, kmem_cache_destroy(cache));
> +}
> +
>  static void kasan_memchr(struct kunit *test)
>  {
>         char *ptr;
> @@ -1183,6 +1193,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
>         KUNIT_CASE(ksize_uaf),
>         KUNIT_CASE(kmem_cache_double_free),
>         KUNIT_CASE(kmem_cache_invalid_free),
> +       KUNIT_CASE(kmem_cache_double_destroy),
>         KUNIT_CASE(kasan_memchr),
>         KUNIT_CASE(kasan_memcmp),
>         KUNIT_CASE(kasan_strings),
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Thanks!