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Message-ID: <CACzwLxhjvJ5WmgB-yxZt3x5YQss9dLhL7KoHra0T-E2jm=vEAQ@mail.gmail.com>
Date: Wed, 2 Oct 2024 21:39:57 +0500
From: Sabyrzhan Tasbolatov <snovitoll@...il.com>
To: Marco Elver <elver@...gle.com>
Cc: ryabinin.a.a@...il.com, glider@...gle.com, andreyknvl@...il.com,
dvyukov@...gle.com, vincenzo.frascino@....com, akpm@...ux-foundation.org,
kasan-dev@...glegroups.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: instrument copy_from/to_kernel_nofault
On Wed, Oct 2, 2024 at 9:00 PM Marco Elver <elver@...gle.com> wrote:
>
> On Fri, 27 Sept 2024 at 17:14, Sabyrzhan Tasbolatov <snovitoll@...il.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().
>
>
> > 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 <andreyknvl@...il.com>
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
> > Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@...il.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
> >
> > --
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