Re: [PATCH v6] mm, kasan, kmsan: copy_from/to_kernel_nofault

[Sabyrzhan Tasbolatov]

On Sun, Oct 13, 2024 at 3:45 AM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
>
> On Fri, Oct 11, 2024 at 5:52 AM Sabyrzhan Tasbolatov
> <snovitoll@xxxxxxxxx> wrote:
> >
> > Instrument copy_from_kernel_nofault() with KMSAN for uninitialized kernel
> > memory check and copy_to_kernel_nofault() with KASAN, KCSAN to detect
> > the memory corruption.
> >
> > 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() explicitly for the memory corruption instrumentation.
>
> For future reference: please write commit messages in a way that is
> readable standalone. I.e. without obscured references to the
> discussions or problems in the previous versions of the patch. It's
> fine to give such references in itself, but you need to give enough
> context in the commit message to make it understandable without
> looking up those discussions.
>
> > copy_to_kernel_nofault() is 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.
> >
> > Link: https://lore.kernel.org/linux-mm/CANpmjNMAVFzqnCZhEity9cjiqQ9CVN1X7qeeeAp_6yKjwKo8iw@xxxxxxxxxxxxxx/
> > Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
> > Reported-by: syzbot+61123a5daeb9f7454599@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=61123a5daeb9f7454599
> > Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
> > Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>
> > ---
> > v2:
> > - squashed previous submitted in -mm tree 2 patches based on Linus tree
> > v3:
> > - moved checks to *_nofault_loop macros per Marco's comments
> > - edited the commit message
> > v4:
> > - replaced Suggested-by with Reviewed-by
> > v5:
> > - addressed Andrey's comment on deleting CONFIG_KASAN_HW_TAGS check in
> >   mm/kasan/kasan_test_c.c
> > - added explanatory comment in kasan_test_c.c
> > - added Suggested-by: Marco Elver back per Andrew's comment.
> > v6:
> > - deleted checks KASAN_TAG_MIN, KASAN_TAG_KERNEL per Andrey's comment.
> > - added empty line before kfree.
> > ---
> >  mm/kasan/kasan_test_c.c | 34 ++++++++++++++++++++++++++++++++++
> >  mm/kmsan/kmsan_test.c   | 17 +++++++++++++++++
> >  mm/maccess.c            | 10 ++++++++--
> >  3 files changed, 59 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> > index a181e4780d9d..716f2cac9708 100644
> > --- a/mm/kasan/kasan_test_c.c
> > +++ b/mm/kasan/kasan_test_c.c
> > @@ -1954,6 +1954,39 @@ static void rust_uaf(struct kunit *test)
> >         KUNIT_EXPECT_KASAN_FAIL(test, kasan_test_rust_uaf());
> >  }
> >
> > +static void copy_to_kernel_nofault_oob(struct kunit *test)
> > +{
> > +       char *ptr;
> > +       char buf[128];
> > +       size_t size = sizeof(buf);
> > +
> > +       /* This test currently fails with the HW_TAGS mode.
> > +        * The reason is unknown and needs to be investigated. */
> > +       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);
> > +
> > +       /*
> > +       * We test copy_to_kernel_nofault() to detect corrupted memory that is
> > +       * being written into the kernel. In contrast, copy_from_kernel_nofault()
> > +       * is primarily used in kernel helper functions where the source address
> > +       * might be random or uninitialized. Applying KASAN instrumentation to
> > +       * copy_from_kernel_nofault() could lead to false positives.
> > +       * By focusing KASAN checks only on copy_to_kernel_nofault(),
> > +       * we ensure that only valid memory is written to the kernel,
> > +       * minimizing the risk of kernel corruption while avoiding
> > +       * false positives in the reverse case.
> > +       */
> > +       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),
> > @@ -2027,6 +2060,7 @@ 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_to_kernel_nofault_oob),
> >         KUNIT_CASE(rust_uaf),
> >         {}
> >  };
> > diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
> > index 13236d579eba..9733a22c46c1 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 518a25667323..3ca55ec63a6a 100644
> > --- a/mm/maccess.c
> > +++ b/mm/maccess.c
> > @@ -13,9 +13,14 @@ bool __weak copy_from_kernel_nofault_allowed(const void *unsafe_src,
> >         return true;
> >  }
> >
> > +/*
> > + * The below only uses kmsan_check_memory() to ensure uninitialized kernel
> > + * memory isn't leaked.
> > + */
> >  #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);        \
> > +               kmsan_check_memory(src, sizeof(type));                  \
> >                 dst += sizeof(type);                                    \
> >                 src += sizeof(type);                                    \
> >                 len -= sizeof(type);                                    \
> > @@ -49,7 +54,8 @@ 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);        \
> > +               instrument_write(dst, sizeof(type));                    \
> >                 dst += sizeof(type);                                    \
> >                 src += sizeof(type);                                    \
> >                 len -= sizeof(type);                                    \
> > --
> > 2.34.1
> >
>
> Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Tested-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
>
> For KASAN parts.

Andrew,

Please let me know if the last v6 is ready for -mm tree.

Previous version was removed here:
https://lore.kernel.org/mm-commits/20241010214955.DBEB7C4CEC5@xxxxxxxxxxxxxxx/

Hopefully, they won't conflict in mm/kasan/kasan_test_c.c per another patch:
https://lore.kernel.org/linux-mm/20241014025701.3096253-3-snovitoll@xxxxxxxxx/

Thanks

>
> Thank you!