[PATCH v2 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 triggers an MTE fault.

Implement a KASAN KUnit test that triggers the issue with the current
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>
Link: https://linux-review.googlesource.com/id/If88e396b9e7c058c1a4b5a252274120e77b1898a
---
v2:
- rebased
- fixed test failure

 mm/kasan/kasan_test_c.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
index 59d673400085f..c4bb3ee497b54 100644
--- a/mm/kasan/kasan_test_c.c
+++ b/mm/kasan/kasan_test_c.c
@@ -1570,7 +1570,9 @@ static void kasan_memcmp(struct kunit *test)
 static void kasan_strings(struct kunit *test)
 {
 	char *ptr;
-	size_t size = 24;
+	char *src, *src2;
+	u8 tag;
+	size_t size = 2 * KASAN_GRANULE_SIZE;
 
 	/*
 	 * str* functions are not instrumented with CONFIG_AMD_MEM_ENCRYPT.
@@ -1581,6 +1583,33 @@ static void kasan_strings(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	src = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
+	strscpy(src, "f0cacc1a00000000f0cacc1a00000000", size);
+
+	tag = get_tag(src);
+
+	src2 = src + KASAN_GRANULE_SIZE;
+
+	/*
+	 * Shorten string and poison the granule after it so that the unaligned
+	 * read in strscpy() triggers a tag mismatch.
+	 */
+	src[KASAN_GRANULE_SIZE - 1] = '\0';
+	kasan_poison(src2, KASAN_GRANULE_SIZE, tag + 1, false);
+
+	/*
+	 * 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, size));
+
+	/* Undo operations above. */
+	src[KASAN_GRANULE_SIZE - 1] = '0';
+	kasan_poison(src2, KASAN_GRANULE_SIZE, tag, false);
+
+	kfree(src);
 	kfree(ptr);
 
 	/*
-- 
2.49.0.395.g12beb8f557-goog