kmsan: introduce test_memcpy_initialized_gap()

Add a regression test for the special case where memcpy() previously
failed to correctly set the origins: if upon memcpy() four aligned
initialized bytes with a zero origin value ended up split between two
aligned four-byte chunks, one of those chunks could've received the zero
origin value even despite it contained uninitialized bytes from other
writes.

Link: https://lkml.kernel.org/r/20230911145702.2663753-4-glider@google.comSigned-off-by: Alexander Potapenko <glider@google.com>
Suggested-by: Marco Elver <elver@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kmsan/kmsan_test.c

@@ -486,6 +486,58 @@ static void test_memcpy_aligned_to_unaligned(struct kunit *test)
 	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
 }
 
+/*
+ * Test case: ensure that origin slots do not accidentally get overwritten with
+ * zeroes during memcpy().
+ *
+ * Previously, when copying memory from an aligned buffer to an unaligned one,
+ * if there were zero origins corresponding to zero shadow values in the source
+ * buffer, they could have ended up being copied to nonzero shadow values in the
+ * destination buffer:
+ *
+ *  memcpy(0xffff888080a00000, 0xffff888080900002, 8)
+ *
+ *  src (0xffff888080900002): ..xx .... xx..
+ *  src origins:              o111 0000 o222
+ *  dst (0xffff888080a00000): xx.. ..xx
+ *  dst origins:              o111 0000
+ *                        (or 0000 o222)
+ *
+ * (here . stands for an initialized byte, and x for an uninitialized one.
+ *
+ * Ensure that this does not happen anymore, and for both destination bytes
+ * the origin is nonzero (i.e. KMSAN reports an error).
+ */
+static void test_memcpy_initialized_gap(struct kunit *test)
+{
+	EXPECTATION_UNINIT_VALUE_FN(expect, "test_memcpy_initialized_gap");
+	volatile char uninit_src[12];
+	volatile char dst[8] = { 0 };
+
+	kunit_info(
+		test,
+		"unaligned 4-byte initialized value gets a nonzero origin after memcpy() - (2 UMR reports)\n");
+
+	uninit_src[0] = 42;
+	uninit_src[1] = 42;
+	uninit_src[4] = 42;
+	uninit_src[5] = 42;
+	uninit_src[6] = 42;
+	uninit_src[7] = 42;
+	uninit_src[10] = 42;
+	uninit_src[11] = 42;
+	memcpy_noinline((void *)&dst[0], (void *)&uninit_src[2], 8);
+
+	kmsan_check_memory((void *)&dst[0], 4);
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
+	report_reset();
+	kmsan_check_memory((void *)&dst[2], 4);
+	KUNIT_EXPECT_FALSE(test, report_matches(&expect));
+	report_reset();
+	kmsan_check_memory((void *)&dst[4], 4);
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
+}
+
 /* Generate test cases for memset16(), memset32(), memset64(). */
 #define DEFINE_TEST_MEMSETXX(size)                                          \
 	static void test_memset##size(struct kunit *test)                   \
@@ -579,6 +631,7 @@ static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_init_memcpy),
 	KUNIT_CASE(test_memcpy_aligned_to_aligned),
 	KUNIT_CASE(test_memcpy_aligned_to_unaligned),
+	KUNIT_CASE(test_memcpy_initialized_gap),
 	KUNIT_CASE(test_memset16),
 	KUNIT_CASE(test_memset32),
 	KUNIT_CASE(test_memset64),