[PATCH] kasan/test: only do kmalloc_double_kzfree for generic mode

[PATCH] kasan/test: only do kmalloc_double_kzfree for generic mode

[Wang Wensheng]

kmalloc_double_kzfree() would corrupt kernel memory when the just
freed memory were allocated by another thread before the second
call to kfree_sensitive().

This could not happen in GENERIC mode as it uses quarantine.

Signed-off-by: Wang Wensheng <wsw9603@163.com>
---
 mm/kasan/kasan_test_c.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
index 32d06cbf6a31..02e43ccc9f21 100644
--- a/mm/kasan/kasan_test_c.c
+++ b/mm/kasan/kasan_test_c.c
@@ -874,6 +874,12 @@ static void kmalloc_double_kzfree(struct kunit *test)
 	char *ptr;
 	size_t size = 16;
 
+	/*
+	 * Only generic KASAN uses quarantine, which could prevent the just freed
+	 * memory from being allocated soon.
+	 */
+	KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_KASAN_GENERIC);
+
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
-- 
2.43.0

Re: [PATCH] kasan/test: only do kmalloc_double_kzfree for generic mode

[Andrey Konovalov]

On Sun, May 17, 2026 at 9:17 AM Wang Wensheng <wsw9603@163.com> wrote:
>
> kmalloc_double_kzfree() would corrupt kernel memory when the just
> freed memory were allocated by another thread before the second
> call to kfree_sensitive().
>
> This could not happen in GENERIC mode as it uses quarantine.

Quite unfortunate to disable this test for HW_TAGS, but I don't see an
easy way to resolve the issue otherwise.

>
> Signed-off-by: Wang Wensheng <wsw9603@163.com>
> ---
>  mm/kasan/kasan_test_c.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> index 32d06cbf6a31..02e43ccc9f21 100644
> --- a/mm/kasan/kasan_test_c.c
> +++ b/mm/kasan/kasan_test_c.c
> @@ -874,6 +874,12 @@ static void kmalloc_double_kzfree(struct kunit *test)
>         char *ptr;
>         size_t size = 16;
>
> +       /*
> +        * Only generic KASAN uses quarantine, which could prevent the just freed
> +        * memory from being allocated soon.
> +        */

Let's reword this to:

With the tag-based KASAN modes, if the memory happens to be
reallocated between the two frees and the new allocation tag happens
to match the old one, the second free will cause a memory corruption.
Resolving https://bugzilla.kernel.org/show_bug.cgi?id=212177 would
help to deal with this. With Generic KASAN, it's effectively
impossible for the memory to get reallocated due to the quarantine.

> +       KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_KASAN_GENERIC);
> +
>         ptr = kmalloc(size, GFP_KERNEL);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>
> --
> 2.43.0
>
> --
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With the comment rewording:

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

Re: [PATCH] kasan/test: only do kmalloc_double_kzfree for generic mode

[wangwensheng]

On 5/21/26 02:19, Andrey Konovalov wrote:
> On Sun, May 17, 2026 at 9:17 AM Wang Wensheng <wsw9603@163.com> wrote:
>> kmalloc_double_kzfree() would corrupt kernel memory when the just
>> freed memory were allocated by another thread before the second
>> call to kfree_sensitive().
>>
>> This could not happen in GENERIC mode as it uses quarantine.
> Quite unfortunate to disable this test for HW_TAGS, but I don't see an
> easy way to resolve the issue otherwise.
>
>> Signed-off-by: Wang Wensheng <wsw9603@163.com>
>> ---
>>   mm/kasan/kasan_test_c.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
>> index 32d06cbf6a31..02e43ccc9f21 100644
>> --- a/mm/kasan/kasan_test_c.c
>> +++ b/mm/kasan/kasan_test_c.c
>> @@ -874,6 +874,12 @@ static void kmalloc_double_kzfree(struct kunit *test)
>>          char *ptr;
>>          size_t size = 16;
>>
>> +       /*
>> +        * Only generic KASAN uses quarantine, which could prevent the just freed
>> +        * memory from being allocated soon.
>> +        */
> Let's reword this to:
>
> With the tag-based KASAN modes, if the memory happens to be
> reallocated between the two frees and the new allocation tag happens
> to match the old one, the second free will cause a memory corruption.
> Resolving https://bugzilla.kernel.org/show_bug.cgi?id=212177 would
> help to deal with this. With Generic KASAN, it's effectively
> impossible for the memory to get reallocated due to the quarantine.
>
Thanks for your nice comment. I have sent a second version.
>> +       KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_KASAN_GENERIC);
>> +
>>          ptr = kmalloc(size, GFP_KERNEL);
>>          KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>>
>> --
>> 2.43.0
>>
>> --
>> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
>> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@googlegroups.com.
>> To view this discussion visit https://groups.google.com/d/msgid/kasan-dev/20260517071713.353579-1-wsw9603%40163.com.
> With the comment rewording:
>
> Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>