[PATCH] mm, kmsan: instrument copy_from_kernel_nofault

From: Sabyrzhan Tasbolatov
Date: Sat Oct 05 2024 - 05:22:39 EST


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@xxxxxxxxxxxxxx/
Suggested-by: Marco Elver <elver@xxxxxxxxxx>
Reported-by: syzbot+61123a5daeb9f7454599@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=61123a5daeb9f7454599
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>
---
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