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

On Tue, Jan 12, 2021 at 2:39 PM Marco Elver [off-list ref] wrote:

On Tue, Jan 05, 2021 at 07:27PM +0100, Andrey Konovalov wrote:

quoted

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 <redacted>
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!