[patch 094/173] kasan: adapt kmalloc_uaf2 test to HW_TAGS mode

From: Andrey Konovalov <redacted>
Subject: kasan: adapt kmalloc_uaf2 test to HW_TAGS mode

In the kmalloc_uaf2() test, the pointers to the two allocated memory
blocks might happen to be the same, and the test will fail. With the
software tag-based mode, the probability of the that 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 16 attempts at generating different tags for the tag-based
modes.

Link: https://linux-review.googlesource.com/id/Ibfa458ef2804ff465d8eb07434a300bf36388d55
Link: https://lkml.kernel.org/r/9cd5cf2f633dcbf55cab801cd26845d2b075cec7.1610733117.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <redacted>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <redacted>
Cc: Branislav Rankov <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <redacted>
Cc: Kevin Brodsky <redacted>
Cc: Peter Collingbourne <redacted>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/test_kasan.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/lib/test_kasan.c~kasan-adapt-kmalloc_uaf2-test-to-hw_tags-mode
+++ a/lib/test_kasan.c

@@ -382,7 +382,9 @@ static void kmalloc_uaf2(struct kunit *t
 {
 	char *ptr1, *ptr2;
 	size_t size = 43;
+	int counter = 0;
 
+again:
 	ptr1 = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
 

@@ -391,6 +393,15 @@ static void kmalloc_uaf2(struct kunit *t
 	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 16 attempts at generating different tags.
+	 */
+	if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 16) {
+		kfree(ptr2);
+		goto again;
+	}
+
 	KUNIT_EXPECT_KASAN_FAIL(test, ptr1[40] = 'x');
 	KUNIT_EXPECT_PTR_NE(test, ptr1, ptr2);
 

_