2021-01-05 18:32:39

by Andrey Konovalov

Subject: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode

In the kmalloc_uaf2() test, the pointers to the two allocated memory
blocks might be the same, and the test will fail. With the software
tag-based mode, the probability of the that happening is 1/254, so it's
hard to observe the failure. For the hardware tag-based mode though,
the probablity is 1/14, which is quite noticable.

Allow up to 4 attempts at generating different tags for the tag-based
modes.

Signed-off-by: Andrey Konovalov <[email protected]>
Link: https://linux-review.googlesource.com/id/Ibfa458ef2804ff465d8eb07434a300bf36388d55
---
lib/test_kasan.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index b5077a47b95a..b67da7f6e17f 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -375,7 +375,9 @@ static void kmalloc_uaf2(struct kunit *test)
{
char *ptr1, *ptr2;
size_t size = 43;
+ int counter = 0;

+again:
ptr1 = kmalloc(size, GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);

@@ -384,6 +386,13 @@ static void kmalloc_uaf2(struct kunit *test)
ptr2 = kmalloc(size, GFP_KERNEL);
KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);

+ /*
+ * For tag-based KASAN ptr1 and ptr2 tags might happen to be the same.
+ * Allow up to 4 attempts at generating different tags.
+ */
+ if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 4)
+ goto again;
+
KUNIT_EXPECT_KASAN_FAIL(test, ptr1[40] = 'x');
KUNIT_EXPECT_PTR_NE(test, ptr1, ptr2);

--
2.29.2.729.g45daf8777d-goog

2021-01-12 13:42:21

by Marco Elver

Subject: Re: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode

On Tue, Jan 05, 2021 at 07:27PM +0100, Andrey Konovalov wrote:
> In the kmalloc_uaf2() test, the pointers to the two allocated memory
> blocks might be the same, and the test will fail. With the software
> tag-based mode, the probability of the that happening is 1/254, so it's
> hard to observe the failure. For the hardware tag-based mode though,
> the probablity is 1/14, which is quite noticable.
>
> Allow up to 4 attempts at generating different tags for the tag-based
> modes.
>
> Signed-off-by: Andrey Konovalov <[email protected]>
> Link: https://linux-review.googlesource.com/id/Ibfa458ef2804ff465d8eb07434a300bf36388d55
> ---
> lib/test_kasan.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index b5077a47b95a..b67da7f6e17f 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -375,7 +375,9 @@ static void kmalloc_uaf2(struct kunit *test)
> {
> char *ptr1, *ptr2;
> size_t size = 43;
> + int counter = 0;
>
> +again:
> ptr1 = kmalloc(size, GFP_KERNEL);
> KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
>
> @@ -384,6 +386,13 @@ static void kmalloc_uaf2(struct kunit *test)
> ptr2 = kmalloc(size, GFP_KERNEL);
> KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);
>
> + /*
> + * For tag-based KASAN ptr1 and ptr2 tags might happen to be the same.
> + * Allow up to 4 attempts at generating different tags.
> + */
> + if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 4)
> + goto again;
> +

Why do we even need a limit? Why not retry until ptr1 != ptr2?

> KUNIT_EXPECT_KASAN_FAIL(test, ptr1[40] = 'x');
> KUNIT_EXPECT_PTR_NE(test, ptr1, ptr2);
>
> --
> 2.29.2.729.g45daf8777d-goog
>

2021-01-12 23:19:44

by Alexander Potapenko

Subject: Re: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode

Nit: s/adopt/adapt in the title.


> +again:
> ptr1 = kmalloc(size, GFP_KERNEL);
> KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
>
> @@ -384,6 +386,13 @@ static void kmalloc_uaf2(struct kunit *test)
> ptr2 = kmalloc(size, GFP_KERNEL);
> KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);
>
> + /*
> + * For tag-based KASAN ptr1 and ptr2 tags might happen to be the same.
> + * Allow up to 4 attempts at generating different tags.
> + */
> + if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 4)
> + goto again;
> +

Looks like we are leaking memory allocated for ptr2 here?

2021-01-13 02:30:39

by Andrey Konovalov

Subject: Re: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode

On Tue, Jan 12, 2021 at 2:39 PM Marco Elver <[email protected]> wrote:
>
> On Tue, Jan 05, 2021 at 07:27PM +0100, Andrey Konovalov wrote:
> > In the kmalloc_uaf2() test, the pointers to the two allocated memory
> > blocks might be the same, and the test will fail. With the software
> > tag-based mode, the probability of the that happening is 1/254, so it's
> > hard to observe the failure. For the hardware tag-based mode though,
> > the probablity is 1/14, which is quite noticable.
> >
> > Allow up to 4 attempts at generating different tags for the tag-based
> > modes.
> >
> > Signed-off-by: Andrey Konovalov <[email protected]>
> > Link: https://linux-review.googlesource.com/id/Ibfa458ef2804ff465d8eb07434a300bf36388d55
> > ---
> > lib/test_kasan.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> > index b5077a47b95a..b67da7f6e17f 100644
> > --- a/lib/test_kasan.c
> > +++ b/lib/test_kasan.c
> > @@ -375,7 +375,9 @@ static void kmalloc_uaf2(struct kunit *test)
> > {
> > char *ptr1, *ptr2;
> > size_t size = 43;
> > + int counter = 0;
> >
> > +again:
> > ptr1 = kmalloc(size, GFP_KERNEL);
> > KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
> >
> > @@ -384,6 +386,13 @@ static void kmalloc_uaf2(struct kunit *test)
> > ptr2 = kmalloc(size, GFP_KERNEL);
> > KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);
> >
> > + /*
> > + * For tag-based KASAN ptr1 and ptr2 tags might happen to be the same.
> > + * Allow up to 4 attempts at generating different tags.
> > + */
> > + if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 4)
> > + goto again;
> > +
>
> Why do we even need a limit? Why not retry until ptr1 != ptr2?

Then the test will hang if it's failing. Let's do up to 16 attempts,
it should be more than enough in practice. Thanks!

2021-01-13 02:32:26

by Andrey Konovalov

Subject: Re: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode

On Tue, Jan 12, 2021 at 9:26 AM Alexander Potapenko <[email protected]> wrote:
>
> Nit: s/adopt/adapt in the title.
>
>
> > +again:
> > ptr1 = kmalloc(size, GFP_KERNEL);
> > KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
> >
> > @@ -384,6 +386,13 @@ static void kmalloc_uaf2(struct kunit *test)
> > ptr2 = kmalloc(size, GFP_KERNEL);
> > KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);
> >
> > + /*
> > + * For tag-based KASAN ptr1 and ptr2 tags might happen to be the same.
> > + * Allow up to 4 attempts at generating different tags.
> > + */
> > + if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 4)
> > + goto again;
> > +
>
> Looks like we are leaking memory allocated for ptr2 here?

Will fix in v2, thanks!