linux-kernel - [PATCH] kmsan: introduce test_unpoison

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Message-Id: <20240524232804.1984355-1-bjohannesmeyer@gmail.com>
Date: Sat, 25 May 2024 01:28:04 +0200
From: Brian Johannesmeyer <bjohannesmeyer@...il.com>
To: Brian Johannesmeyer <bjohannesmeyer@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kasan-dev@...glegroups.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] kmsan: introduce test_unpoison_memory()

Add a regression test to ensure that kmsan_unpoison_memory() works the same
as an unpoisoning operation added by the instrumentation. (Of course,
please correct me if I'm misunderstanding how these should work).

The test has two subtests: one that checks the instrumentation, and one
that checks kmsan_unpoison_memory(). Each subtest initializes the first
byte of a 4-byte buffer, then checks that the other 3 bytes are
uninitialized. Unfortunately, the test for kmsan_unpoison_memory() fails to
identify the 3 bytes as uninitialized (i.e., the line with the comment
"Fail: No UMR report").

As to my guess why this is happening: From kmsan_unpoison_memory(), the
backing shadow is indeed correctly overwritten in
kmsan_internal_set_shadow_origin() via `__memset(shadow_start, b, size);`.
Instead, the issue seems to stem from overwriting the backing origin, in
the following `origin_start[i] = origin;` loop; if we return before that
loop on this specific call to kmsan_unpoison_memory(), then the test
passes.

Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@...il.com>
---
 mm/kmsan/kmsan_test.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
index 07d3a3a5a9c5..c3ab90df0abf 100644
--- a/mm/kmsan/kmsan_test.c
+++ b/mm/kmsan/kmsan_test.c
@@ -614,6 +614,30 @@ static void test_stackdepot_roundtrip(struct kunit *test)
 	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
 }

+/*
+ * Test case: ensure that kmsan_unpoison_memory() and the instrumentation work
+ * the same
+ */
+static void test_unpoison_memory(struct kunit *test)
+{
+	EXPECTATION_UNINIT_VALUE_FN(expect, "test_unpoison_memory");
+	volatile char a[4], b[4];
+
+	kunit_info(
+		test,
+		"unpoisoning via the instrumentation vs. kmsan_unpoison_memory() (2 UMR reports)\n");
+
+	a[0] = 0;                                     // Initialize a[0]
+	kmsan_check_memory((char *)&a[1], 3);         // Check a[1]--a[3]
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect)); // Pass: UMR report
+
+	report_reset();
+
+	kmsan_unpoison_memory((char *)&b[0], 1);  // Initialize b[0]
+	kmsan_check_memory((char *)&b[1], 3);     // Check b[1]--b[3]
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect)); // Fail: No UMR report
+}
+
 static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_uninit_kmalloc),
 	KUNIT_CASE(test_init_kmalloc),
@@ -637,6 +661,7 @@ static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_memset64),
 	KUNIT_CASE(test_long_origin_chain),
 	KUNIT_CASE(test_stackdepot_roundtrip),
+	KUNIT_CASE(test_unpoison_memory),
 	{},
 };

-- 
2.34.1