Re: [PATCH] mm: instrument copy_from/to_kernel_nofault

[Sabyrzhan Tasbolatov]

On Wed, Oct 2, 2024 at 9:00 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Fri, 27 Sept 2024 at 17:14, Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx> 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@xxxxxxxxx>
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
> > Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>
> > ---
> >  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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