[PATCH] mm: instrument copy_from/to_kernel_nofault

[Sabyrzhan Tasbolatov]

Instrument copy_from_kernel_nofault(), copy_to_kernel_nofault()
with instrument_memcpy_before() for KASAN, KCSAN checks and
instrument_memcpy_after() for KMSAN.

Tested on x86_64 and arm64 with CONFIG_KASAN_SW_TAGS.
On arm64 with CONFIG_KASAN_HW_TAGS, kunit test currently fails.
Need more clarification on it - currently, disabled in kunit test.

Reported-by: Andrey Konovalov <andre...@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
Signed-off-by: Sabyrzhan Tasbolatov <snov...@gmail.com>
---
mm/kasan/kasan_test.c | 31 +++++++++++++++++++++++++++++++
mm/maccess.c | 8 ++++++--
2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
index 567d33b49..329d81518 100644
--- a/mm/kasan/kasan_test.c
+++ b/mm/kasan/kasan_test.c
@@ -1944,6 +1944,36 @@ static void match_all_mem_tag(struct kunit *test)
kfree(ptr);
}

+static void copy_from_to_kernel_nofault_oob(struct kunit *test)
+{
+ char *ptr;
+ char buf[128];
+ size_t size = sizeof(buf);
+
+ /* Not detecting fails currently with HW_TAGS */
+ KASAN_TEST_NEEDS_CONFIG_OFF(test, CONFIG_KASAN_HW_TAGS);
+
+ ptr = kmalloc(size - KASAN_GRANULE_SIZE, GFP_KERNEL);
+ KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
+ OPTIMIZER_HIDE_VAR(ptr);
+
+ if (IS_ENABLED(CONFIG_KASAN_SW_TAGS)) {
+ /* Check that the returned pointer is tagged. */
+ KUNIT_EXPECT_GE(test, (u8)get_tag(ptr), (u8)KASAN_TAG_MIN);
+ 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,
+ copy_to_kernel_nofault(ptr, &buf[0], size));
+ kfree(ptr);
+}
+
static struct kunit_case kasan_kunit_test_cases[] = {
KUNIT_CASE(kmalloc_oob_right),
KUNIT_CASE(kmalloc_oob_left),
@@ -2017,6 +2047,7 @@ 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),
{}
};

diff --git a/mm/maccess.c b/mm/maccess.c
index 518a25667..2c4251df4 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -15,7 +15,7 @@ bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,

#define copy_from_kernel_nofault_loop(dst, src, len, type, err_label) \
while (len >= sizeof(type)) { \
- __get_kernel_nofault(dst, src, type, err_label); \
+ __get_kernel_nofault(dst, src, type, err_label); \
dst += sizeof(type); \
src += sizeof(type); \
len -= sizeof(type); \
@@ -32,6 +32,7 @@ long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
return -ERANGE;

pagefault_disable();
+ instrument_memcpy_before(dst, src, size);
if (!(align & 7))
copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
if (!(align & 3))
@@ -39,6 +40,7 @@ long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
if (!(align & 1))
copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
+ instrument_memcpy_after(dst, src, size, 0);
pagefault_enable();
return 0;
Efault:
@@ -49,7 +51,7 @@ EXPORT_SYMBOL_GPL(copy_from_kernel_nofault);

#define copy_to_kernel_nofault_loop(dst, src, len, type, err_label) \
while (len >= sizeof(type)) { \
- __put_kernel_nofault(dst, src, type, err_label); \
+ __put_kernel_nofault(dst, src, type, err_label); \
dst += sizeof(type); \
src += sizeof(type); \
len -= sizeof(type); \
@@ -63,6 +65,7 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
align = (unsigned long)dst | (unsigned long)src;

pagefault_disable();
+ instrument_memcpy_before(dst, src, size);
if (!(align & 7))
copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
if (!(align & 3))
@@ -70,6 +73,7 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
if (!(align & 1))
copy_to_kernel_nofault_loop(dst, src, size, u16, Efault);
copy_to_kernel_nofault_loop(dst, src, size, u8, Efault);
+ instrument_memcpy_after(dst, src, size, 0);
pagefault_enable();
return 0;
Efault:
--
2.34.1

[Marco Elver]

On Fri, 27 Sept 2024 at 17:14, Sabyrzhan Tasbolatov <snov...@gmail.com> wrote:
>
> Instrument copy_from_kernel_nofault(), copy_to_kernel_nofault()
> with instrument_memcpy_before() for KASAN, KCSAN checks and
> instrument_memcpy_after() for KMSAN.

There's a fundamental problem with instrumenting
copy_from_kernel_nofault() - it's meant to be a non-faulting helper,
i.e. if it attempts to read arbitrary kernel addresses, that's not a
problem because it won't fault and BUG. These may be used in places
that probe random memory, and KASAN may say that some memory is
invalid and generate a report - but in reality that's not a problem.

In the Bugzilla bug, Andrey wrote:

> KASAN should check both arguments of copy_from/to_kernel_nofault() for accessibility when both are fault-safe.

I don't see this patch doing it, or at least it's not explained. By
looking at the code, I see that it does the instrument_memcpy_before()
right after pagefault_disable(), which tells me that KASAN or other
tools will complain if a page is not faulted in. These helpers are
meant to be usable like that - despite their inherent unsafety,
there's little that I see that KASAN can help with.

What _might_ be useful, is detecting copying faulted-in but
uninitialized memory to user space. So I think the only
instrumentation we want to retain is KMSAN instrumentation for the
copy_from_kernel_nofault() helper, and only if no fault was
encountered.

Instrumenting copy_to_kernel_nofault() may be helpful to catch memory
corruptions, but only if faulted-in memory was accessed.

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[Sabyrzhan Tasbolatov]

On Wed, Oct 2, 2024 at 9:00 PM Marco Elver <el...@google.com> wrote:
>
> On Fri, 27 Sept 2024 at 17:14, Sabyrzhan Tasbolatov <snov...@gmail.com> wrote:
> >
> > Instrument copy_from_kernel_nofault(), copy_to_kernel_nofault()
> > with instrument_memcpy_before() for KASAN, KCSAN checks and
> > instrument_memcpy_after() for KMSAN.
>
> There's a fundamental problem with instrumenting
> copy_from_kernel_nofault() - it's meant to be a non-faulting helper,
> i.e. if it attempts to read arbitrary kernel addresses, that's not a
> problem because it won't fault and BUG. These may be used in places
> that probe random memory, and KASAN may say that some memory is
> invalid and generate a report - but in reality that's not a problem.
>
> In the Bugzilla bug, Andrey wrote:
>
> > KASAN should check both arguments of copy_from/to_kernel_nofault() for accessibility when both are fault-safe.
>
> I don't see this patch doing it, or at least it's not explained. By
> looking at the code, I see that it does the instrument_memcpy_before()
> right after pagefault_disable(), which tells me that KASAN or other
> tools will complain if a page is not faulted in. These helpers are
> meant to be usable like that - despite their inherent unsafety,
> there's little that I see that KASAN can help with.

Hello, thanks for the comment!
instrument_memcpy_before() has been replaced with
instrument_read() and instrument_write() in
commit 9e3f2b1ecdd4("mm, kasan: proper instrument _kernel_nofault"),
and there are KASAN, KCSAN checks.

> What _might_ be useful, is detecting copying faulted-in but
> uninitialized memory to user space. So I think the only
> instrumentation we want to retain is KMSAN instrumentation for the
> copy_from_kernel_nofault() helper, and only if no fault was
> encountered.
>
> Instrumenting copy_to_kernel_nofault() may be helpful to catch memory
> corruptions, but only if faulted-in memory was accessed.

If we need to have KMSAN only instrumentation for
copy_from_user_nofault(), then AFAIU, in mm/kasan/kasan_test.c
copy_from_to_kernel_nofault_oob() should have only
copy_to_kernel_nofault() OOB kunit test to trigger KASAN.
And copy_from_user_nofault() kunit test can be placed in mm/kmsan/kmsan_test.c.

I wonder if instrument_get_user macro is OK for src ptr in
copy_from_kernel_nofault().

If this is true understanding, then there is no need to add
kasan_disable_current(),
kasan_enable_current() for kernel helpers functions that use
copy_from_kernel_nofault().

[Marco Elver]

Did you mean s/copy_from_user_nofault/copy_from_kernel_nofault/?

> copy_from_to_kernel_nofault_oob() should have only
> copy_to_kernel_nofault() OOB kunit test to trigger KASAN.
> And copy_from_user_nofault() kunit test can be placed in mm/kmsan/kmsan_test.c.

I think in the interest of reducing false positives, I'd proceed with
making copy_from_kernel_nofault() KMSAN only.

[Sabyrzhan Tasbolatov]

Yes, typo, sorry for the confusion.

>
> > copy_from_to_kernel_nofault_oob() should have only
> > copy_to_kernel_nofault() OOB kunit test to trigger KASAN.
> > And copy_from_user_nofault() kunit test can be placed in mm/kmsan/kmsan_test.c.
>
> I think in the interest of reducing false positives, I'd proceed with
> making copy_from_kernel_nofault() KMSAN only.

Here is my current upcoming patch that I will send separately
once it's tested, it's slowly being compiled on my laptop (away from PC).
I've moved copy_from_kernel_nofault() to kmsan_test.c and added
kmsan_check_memory() _before_ pagefault_disable() to check
kernel src address if it's initialized.
For copy_to_kernel_nofault() , I've left instrument_write() for memory
corruption check but before pagefault_disable() again, if I understood the logic
correctly. I will adjust kmsan kunit test once I can run it and send a PATCH.
Meanwhile, please let me know if the order of instrumentation before
pagefault_disable()
is correct.

> By looking at the code, I see that it does the instrument_memcpy_before()
> right after pagefault_disable(), which tells me that KASAN or other
> tools will complain if a page is not faulted in. These helpers are
> meant to be usable like that - despite their inherent unsafety,
> there's little that I see that KASAN can help with.

---
mm/kasan/kasan_test_c.c | 8 ++------
mm/kmsan/kmsan_test.c | 16 ++++++++++++++++
mm/maccess.c | 5 +++--
3 files changed, 21 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..fc50d0aef47 100644
--- a/mm/kmsan/kmsan_test.c
+++ b/mm/kmsan/kmsan_test.c
@@ -640,6 +640,21 @@ 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;
+ volatile char src[4], dst[4];
+
+ EXPECTATION_UNINIT_VALUE_FN(expect, "test_copy_from_kernel_nofault");
+ kunit_info(
+ test,
+ "testing copy_from_kernel_nofault with src uninitialized memory\n");
+
+ ret = copy_from_kernel_nofault(dst, src, sizeof(src));
+ 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 +679,7 @@ 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