[PATCH] mm, kmsan: instrument copy_from_kernel_nofault

[Sabyrzhan Tasbolatov <snovitoll@gmail.com>]

syzbot reported that bpf_probe_read_kernel() kernel helper triggered
KASAN report via kasan_check_range() which is not the expected behaviour
as copy_from_kernel_nofault() is meant to be a non-faulting helper.

Solution is, suggested by Marco Elver, to replace KASAN, KCSAN check in
copy_from_kernel_nofault() with KMSAN detection of copying uninitilaized
kernel memory. In copy_to_kernel_nofault() we can retain
instrument_write() for the memory corruption instrumentation but before
pagefault_disable().

Added KMSAN and modified KASAN kunit tests and tested on x86_64.

This is the part of PATCH series attempting to properly address bugzilla
issue.

Link: https://lore.kernel.org/linux-mm/CANpmjNMAVFzqnCZhEity9cjiqQ9CVN1X7qeeeAp_6yKjwKo8iw@mail.gmail.com/
Suggested-by: Marco Elver <elver@google.com>
Reported-by: syzbot+61123a5daeb9f7454599@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=61123a5daeb9f7454599
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
---
 mm/kasan/kasan_test_c.c |  8 ++------
 mm/kmsan/kmsan_test.c   | 17 +++++++++++++++++
 mm/maccess.c            |  5 +++--
 3 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
index 0a226ab032d..5cff90f831d 100644
--- a/mm/kasan/kasan_test_c.c
+++ b/mm/kasan/kasan_test_c.c
@@ -1954,7 +1954,7 @@ static void rust_uaf(struct kunit *test)
 	KUNIT_EXPECT_KASAN_FAIL(test, kasan_test_rust_uaf());
 }
 
-static void copy_from_to_kernel_nofault_oob(struct kunit *test)
+static void copy_to_kernel_nofault_oob(struct kunit *test)
 {
 	char *ptr;
 	char buf[128];
@@ -1973,10 +1973,6 @@ static void copy_from_to_kernel_nofault_oob(struct kunit *test)
 		KUNIT_EXPECT_LT(test, (u8)get_tag(ptr), (u8)KASAN_TAG_KERNEL);
 	}
 
-	KUNIT_EXPECT_KASAN_FAIL(test,
-		copy_from_kernel_nofault(&buf[0], ptr, size));
-	KUNIT_EXPECT_KASAN_FAIL(test,
-		copy_from_kernel_nofault(ptr, &buf[0], size));
 	KUNIT_EXPECT_KASAN_FAIL(test,
 		copy_to_kernel_nofault(&buf[0], ptr, size));
 	KUNIT_EXPECT_KASAN_FAIL(test,
@@ -2057,7 +2053,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(match_all_not_assigned),
 	KUNIT_CASE(match_all_ptr_tag),
 	KUNIT_CASE(match_all_mem_tag),
-	KUNIT_CASE(copy_from_to_kernel_nofault_oob),
+	KUNIT_CASE(copy_to_kernel_nofault_oob),
 	KUNIT_CASE(rust_uaf),
 	{}
 };
diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
index 13236d579eb..9733a22c46c 100644
--- a/mm/kmsan/kmsan_test.c
+++ b/mm/kmsan/kmsan_test.c
@@ -640,6 +640,22 @@ static void test_unpoison_memory(struct kunit *test)
 	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
 }
 
+static void test_copy_from_kernel_nofault(struct kunit *test)
+{
+	long ret;
+	char buf[4], src[4];
+	size_t size = sizeof(buf);
+
+	EXPECTATION_UNINIT_VALUE_FN(expect, "copy_from_kernel_nofault");
+	kunit_info(
+		test,
+		"testing copy_from_kernel_nofault with uninitialized memory\n");
+
+	ret = copy_from_kernel_nofault((char *)&buf[0], (char *)&src[0], size);
+	USE(ret);
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
+}
+
 static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_uninit_kmalloc),
 	KUNIT_CASE(test_init_kmalloc),
@@ -664,6 +680,7 @@ static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_long_origin_chain),
 	KUNIT_CASE(test_stackdepot_roundtrip),
 	KUNIT_CASE(test_unpoison_memory),
+	KUNIT_CASE(test_copy_from_kernel_nofault),
 	{},
 };
 
diff --git a/mm/maccess.c b/mm/maccess.c
index f752f0c0fa3..a91a39a56cf 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -31,8 +31,9 @@ long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
 	if (!copy_from_kernel_nofault_allowed(src, size))
 		return -ERANGE;
 
+	/* Make sure uninitialized kernel memory isn't copied. */
+	kmsan_check_memory(src, size);
 	pagefault_disable();
-	instrument_read(src, size);
 	if (!(align & 7))
 		copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
 	if (!(align & 3))
@@ -63,8 +64,8 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
 	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
 		align = (unsigned long)dst | (unsigned long)src;
 
-	pagefault_disable();
 	instrument_write(dst, size);
+	pagefault_disable();
 	if (!(align & 7))
 		copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
 	if (!(align & 3))
-- 
2.34.1