[PATCH v4 2/2] kasan: Add strscpy() test to trigger tag fault on arm64

[Peter Collingbourne <pcc@google.com>]

From: Vincenzo Frascino <vincenzo.frascino@arm.com>

When we invoke strscpy() with a maximum size of N bytes, it assumes
that:
- It can always read N bytes from the source.
- It always write N bytes (zero-padded) to the destination.

On aarch64 with Memory Tagging Extension enabled if we pass an N that is
bigger then the source buffer, it would previously trigger an MTE fault.

Implement a KASAN KUnit test that triggers the issue with the previous
implementation of read_word_at_a_time() on aarch64 with MTE enabled.

Cc: Will Deacon <will@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Co-developed-by: Peter Collingbourne <pcc@google.com>
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://linux-review.googlesource.com/id/If88e396b9e7c058c1a4b5a252274120e77b1898a
---
v4:
- clarify commit message
- improve comment

v3:
- simplify test case

v2:
- rebased
- fixed test failure

 mm/kasan/kasan_test_c.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
index 59d673400085f..655356df71fe6 100644
--- a/mm/kasan/kasan_test_c.c
+++ b/mm/kasan/kasan_test_c.c
@@ -1570,6 +1570,7 @@ static void kasan_memcmp(struct kunit *test)
 static void kasan_strings(struct kunit *test)
 {
 	char *ptr;
+	char *src;
 	size_t size = 24;
 
 	/*
@@ -1581,6 +1582,21 @@ static void kasan_strings(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	src = kmalloc(KASAN_GRANULE_SIZE, GFP_KERNEL | __GFP_ZERO);
+	strscpy(src, "f0cacc1a0000000", KASAN_GRANULE_SIZE);
+
+	/*
+	 * Make sure that strscpy() does not trigger KASAN if it overreads into
+	 * poisoned memory.
+	 *
+	 * The expected size does not include the terminator '\0'
+	 * so it is (KASAN_GRANULE_SIZE - 2) ==
+	 * KASAN_GRANULE_SIZE - ("initial removed character" + "\0").
+	 */
+	KUNIT_EXPECT_EQ(test, KASAN_GRANULE_SIZE - 2,
+			strscpy(ptr, src + 1, KASAN_GRANULE_SIZE));
+
+	kfree(src);
 	kfree(ptr);
 
 	/*
-- 
2.49.0.472.ge94155a9ec-goog