Re: [PATCH v2 2/2] kasan: Add strscpy() test to trigger tag fault on arm64

[Peter Collingbourne]

On Thu, Mar 20, 2025 at 10:25 AM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
>
> On Tue, Mar 18, 2025 at 10:41 PM Peter Collingbourne <pcc@xxxxxxxxxx> wrote:
> >
> > From: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> >
> > When we invoke strscpy() with a maximum size of N bytes, it assumes
> > that:
> > - It can always read N bytes from the source.
> > - It always write N bytes (zero-padded) to the destination.
> >
> > On aarch64 with Memory Tagging Extension enabled if we pass an N that is
> > bigger then the source buffer, it triggers an MTE fault.
> >
> > Implement a KASAN KUnit test that triggers the issue with the current
> > implementation of read_word_at_a_time() on aarch64 with MTE enabled.
> >
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> > Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Co-developed-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> > Link: https://linux-review.googlesource.com/id/If88e396b9e7c058c1a4b5a252274120e77b1898a
> > ---
> > v2:
> > - rebased
> > - fixed test failure
> >
> >  mm/kasan/kasan_test_c.c | 31 ++++++++++++++++++++++++++++++-
> >  1 file changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> > index 59d673400085f..c4bb3ee497b54 100644
> > --- a/mm/kasan/kasan_test_c.c
> > +++ b/mm/kasan/kasan_test_c.c
> > @@ -1570,7 +1570,9 @@ static void kasan_memcmp(struct kunit *test)
> >  static void kasan_strings(struct kunit *test)
> >  {
> >         char *ptr;
> > -       size_t size = 24;
> > +       char *src, *src2;
> > +       u8 tag;
> > +       size_t size = 2 * KASAN_GRANULE_SIZE;
> >
> >         /*
> >          * str* functions are not instrumented with CONFIG_AMD_MEM_ENCRYPT.
> > @@ -1581,6 +1583,33 @@ static void kasan_strings(struct kunit *test)
> >         ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
> >         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> >
> > +       src = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
> > +       strscpy(src, "f0cacc1a00000000f0cacc1a00000000", size);
> > +
> > +       tag = get_tag(src);
> > +
> > +       src2 = src + KASAN_GRANULE_SIZE;
> > +
> > +       /*
> > +        * Shorten string and poison the granule after it so that the unaligned
> > +        * read in strscpy() triggers a tag mismatch.
> > +        */
> > +       src[KASAN_GRANULE_SIZE - 1] = '\0';
> > +       kasan_poison(src2, KASAN_GRANULE_SIZE, tag + 1, false);
> > +
> > +       /*
> > +        * The expected size does not include the terminator '\0'
> > +        * so it is (KASAN_GRANULE_SIZE - 2) ==
> > +        * KASAN_GRANULE_SIZE - ("initial removed character" + "\0").
> > +        */
> > +       KUNIT_EXPECT_EQ(test, KASAN_GRANULE_SIZE - 2,
> > +                       strscpy(ptr, src + 1, size));
> > +
> > +       /* Undo operations above. */
> > +       src[KASAN_GRANULE_SIZE - 1] = '0';
> > +       kasan_poison(src2, KASAN_GRANULE_SIZE, tag, false);
> > +
> > +       kfree(src);
>
> I have trouble understanding what this code is doing...
>
> So the goal is to call strcpy with such an address, that the first 8
> bytes (partially) cover 2 granules, one accessible and the other is
> not?

The first 16 bytes, but yes.

> If so, can we not do something like:
>
> src = kmalloc(KASAN_GRANULE_SIZE, GFP_KERNEL | __GFP_ZERO);
> strscpy(src, "aabbcceeddeeffg\0", size);
> strscpy(ptr, src + KASAN_GRANULE_SIZE - 2, sizeof(unsigned long));

Yes, something like that should work as well. Let me send a v3.

Peter

> Otherwise, this code needs more explanatory comments and it's probably
> better to move it out to a helper function.
>
> >         kfree(ptr);
> >
> >         /*
> > --
> > 2.49.0.395.g12beb8f557-goog
> >