Re: [PATCH v3] mm, kasan, kmsan: copy_from/to_kernel_nofault

[Marco Elver <elver@google.com>]

On Tue, 8 Oct 2024 at 12:14, Sabyrzhan Tasbolatov <snovitoll@gmail.com> 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.
>
> 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
> - currently, disabled in kunit test.

I assume you retested. Did you also test the bpf_probe_read_kernel()
false positive no longer appears?

> Link: https://lore.kernel.org/linux-mm/CANpmjNMAVFzqnCZhEity9cjiqQ9CVN1X7qeeeAp_6yKjwKo8iw@mail.gmail.com/
> Suggested-by: Marco Elver <elver@google.com>

This looks more reasonable:

Reviewed-by: Marco Elver <elver@google.com>

This looks like the most conservative thing to do for now.

> Reported-by: syzbot+61123a5daeb9f7454599@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=61123a5daeb9f7454599
> Reported-by: Andrey Konovalov <andreyknvl@gmail.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=210505
> Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
> ---
> 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
> ---
>  mm/kasan/kasan_test_c.c | 27 +++++++++++++++++++++++++++
>  mm/kmsan/kmsan_test.c   | 17 +++++++++++++++++
>  mm/maccess.c            | 10 ++++++++--
>  3 files changed, 52 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> index a181e4780d9d..5cff90f831db 100644
> --- a/mm/kasan/kasan_test_c.c
> +++ b/mm/kasan/kasan_test_c.c
> @@ -1954,6 +1954,32 @@ 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);
> +
> +       /* 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_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 +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_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
>