linux-kernel - Re: [PATCH v3 7/7] selftests/mm: add more mseal traversal tests

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Message-ID: <CAKbZUD3Siwq4GZdOy-2n_txG2BMQ=m7PypB53sQxeLcBE4xYGA@mail.gmail.com>
Date: Wed, 21 Aug 2024 17:20:04 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Jeff Xu <jeffxu@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Shuah Khan <shuah@...nel.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, oliver.sang@...el.com, 
	torvalds@...ux-foundation.org, Michael Ellerman <mpe@...erman.id.au>, 
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 7/7] selftests/mm: add more mseal traversal tests

On Wed, Aug 21, 2024 at 4:56 PM Jeff Xu <jeffxu@...omium.org> wrote:
>
> Hi Pedro
>
> On Fri, Aug 16, 2024 at 5:18 PM Pedro Falcato <pedro.falcato@...il.com> wrote:
> >
> > Add more mseal traversal tests across VMAs, where we could possibly
> > screw up sealing checks. These test more across-vma traversal for
> > mprotect, munmap and madvise. Particularly, we test for the case where a
> > regular VMA is followed by a sealed VMA.
> >
> > Signed-off-by: Pedro Falcato <pedro.falcato@...il.com>
> > ---
> >  tools/testing/selftests/mm/mseal_test.c | 111 +++++++++++++++++++++++++++++++-
> >  1 file changed, 110 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/mm/mseal_test.c b/tools/testing/selftests/mm/mseal_test.c
> > index 259bef4945e9..0d4d40fb0f88 100644
> > --- a/tools/testing/selftests/mm/mseal_test.c
> > +++ b/tools/testing/selftests/mm/mseal_test.c
> > @@ -766,6 +766,42 @@ static void test_seal_mprotect_partial_mprotect(bool seal)
> >         REPORT_TEST_PASS();
> >  }
> >
> > +static void test_seal_mprotect_partial_mprotect_tail(bool seal)
> > +{
> > +       void *ptr;
> > +       unsigned long page_size = getpagesize();
> > +       unsigned long size = 2 * page_size;
> > +       int ret;
> > +       int prot;
> > +
> > +       /*
> > +        * Check if a partial mseal (that results in two vmas) works correctly.
> > +        * It might mprotect the first, but it'll never touch the second (msealed) vma.
> > +        */
> > +
> > +       setup_single_address(size, &ptr);
> > +       FAIL_TEST_IF_FALSE(ptr != (void *)-1);
> > +
> > +       if (seal) {
> > +               ret = sys_mseal(ptr + page_size, size);
> you are allocating 2 pages , and I assume you are sealing the second
> page, so the size should be page_size.
> ret = sys_mseal(ptr + page_size, page_size);

Yes, good catch, it appears to be harmless but ofc down to straight luck.
I'll send a fixup for this and the other mistake down there.

>
> > +               FAIL_TEST_IF_FALSE(!ret);
> > +       }
> > +
> > +       ret = sys_mprotect(ptr, size, PROT_EXEC);
> > +       if (seal)
> > +               FAIL_TEST_IF_FALSE(ret < 0);
> > +       else
> > +               FAIL_TEST_IF_FALSE(!ret);
> > +
> > +       if (seal) {
> > +               FAIL_TEST_IF_FALSE(get_vma_size(ptr + page_size, &prot) > 0);
> > +               FAIL_TEST_IF_FALSE(prot == 0x4);
> To test partial mprotect, the test needs to add the check for the
> first page to be changed, Also to avoid the merge,  a PROT_NONE page
> can to be added in front.

No, I'm leaving partial mprotect to be undefined. It doesn't make
sense to constraint ourselves, since POSIX wording is already loose.

>
> > +       }
> > +
> > +       REPORT_TEST_PASS();
> > +}
> > +
> > +
> >  static void test_seal_mprotect_two_vma_with_gap(bool seal)
> >  {
> >         void *ptr;
> > @@ -983,6 +1019,41 @@ static void test_seal_munmap_vma_with_gap(bool seal)
> >         REPORT_TEST_PASS();
> >  }
> >
> > +static void test_seal_munmap_partial_across_vmas(bool seal)
> > +{
> > +       void *ptr;
> > +       unsigned long page_size = getpagesize();
> > +       unsigned long size = 2 * page_size;
> > +       int ret;
> > +       int prot;
> > +
> > +       /*
> > +        * Check if a partial mseal (that results in two vmas) works correctly.
> > +        * It might unmap the first, but it'll never unmap the second (msealed) vma.
> > +        */
> > +
> > +       setup_single_address(size, &ptr);
> > +       FAIL_TEST_IF_FALSE(ptr != (void *)-1);
> > +
> > +       if (seal) {
> > +               ret = sys_mseal(ptr + page_size, size);
> ret = sys_mseal(ptr + page_size, page_size);
>
> > +               FAIL_TEST_IF_FALSE(!ret);
> > +       }
> > +
> > +       ret = sys_munmap(ptr, size);
> > +       if (seal)
> > +               FAIL_TEST_IF_FALSE(ret < 0);
> > +       else
> > +               FAIL_TEST_IF_FALSE(!ret);
> > +
> > +       if (seal) {
> > +               FAIL_TEST_IF_FALSE(get_vma_size(ptr + page_size, &prot) > 0);
> > +               FAIL_TEST_IF_FALSE(prot == 0x4);
> To test partial unmap, the test needs to add the check for the first
> page to be freed, Also to avoid the merge,  a PROT_NONE page needs to
> be in front.

I'm not testing partial unmap. Partial unmap does not happen. I have
told you this before.

>
> The test_seal_munmap_partial_across_vmas  shows the behavior
> difference with in-loop approach and out-loop approach. Previously,
> both VMAs will not be freed, now the first VMA will be freed, and the
> second VMA (sealed) won't.
>
> This brings to the line you previously mentioned: [1] and I quote:
> "munmap is atomic and always has been. It's required by POSIX."

This is still true, the comment was a copy-and-paste mindslip. Please
read the email thread. It has been fixed up by Andrew.

-- 
Pedro