Message-ID: <fd927106-2fc3-4b96-8014-2c517229bc99@lucifer.local>
Date: Fri, 30 Aug 2024 20:22:51 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: jeffxu@...omium.org
Cc: akpm@...ux-foundation.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, pedro.falcato@...il.com,
        willy@...radead.org, broonie@...nel.org, vbabka@...e.cz,
        Liam.Howlett@...cle.com, rientjes@...gle.com, keescook@...omium.org
Subject: Re: [PATCH v3 4/5] selftests/mseal: add more tests for mmap

On Fri, Aug 30, 2024 at 07:43:12PM GMT, Lorenzo Stoakes wrote:
> On Fri, Aug 30, 2024 at 06:02:36PM GMT, jeffxu@...omium.org wrote:
> > From: Jeff Xu <jeffxu@...omium.org>
> >
> > Add sealing test to cover mmap for
> > Expand/shrink across sealed vmas (MAP_FIXED)
> > Reuse the same address in !MAP_FIXED case.
>
> This commit message is woefully small. I told you on v1 to improve the
> commit messages. Linus has told you to do this before.
>
> Please actually respond to feedback. Thanks.
>
> >
> > Signed-off-by: Jeff Xu <jeffxu@...omium.org>
> > ---
> >  tools/testing/selftests/mm/mseal_test.c | 126 +++++++++++++++++++++++-
> >  1 file changed, 125 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/mm/mseal_test.c b/tools/testing/selftests/mm/mseal_test.c
> > index e855c8ccefc3..3516389034a7 100644
> > --- a/tools/testing/selftests/mm/mseal_test.c
> > +++ b/tools/testing/selftests/mm/mseal_test.c
> > @@ -2222,6 +2222,123 @@ static void test_munmap_free_multiple_ranges(bool seal)
> >  	REPORT_TEST_PASS();
> >  }
> >
> > +static void test_seal_mmap_expand_seal_middle(bool seal)
>
> This test doesn't expand, doesn't do anything in the middle. It does mmap()
> though and relates to mseal, so that's something... this is compeltely
> misnamed and needs to be rethought.
>

OK correction - it _seals_ in the middle. The remained of the criticism remains,
and this is rather confusing... and I continue to wonder what the purpose of
this is?

> > +{
> > +	void *ptr;
> > +	unsigned long page_size = getpagesize();
> > +	unsigned long size = 12 * page_size;
> > +	int ret;
> > +	void *ret2;
> > +	int prot;
> > +
> > +	setup_single_address(size, &ptr);
>
> Please replace every single instance of this with an mmap(). There's
> literally no reason to abstract it. And munmap() what you map.
>
> > +	FAIL_TEST_IF_FALSE(ptr != (void *)-1);
>
> Pretty sure Pedro pointed out you should be checking against MAP_FAILED
> here. I really don't understand why the rest of your test is full of
> mmap()'s but for some reason you choose to abstract this one call? What?
>
> > +	/* ummap last 4 pages. */
> > +	ret = sys_munmap(ptr + 8 * page_size, 4 * page_size);
>
> sys_munmap()? What's wrong with munmap()?
>
> > +	FAIL_TEST_IF_FALSE(!ret);
>
> Why do we not have a FAIL_TEST_IF_TRUE()? This is crazy.
>
> Would be nice to have something human-readable like ASSERT_EQ() or
> ASSERT_TRUE() or ASSERT_FALSE().
>
> > +
> > +	size = get_vma_size(ptr, &prot);
> > +	FAIL_TEST_IF_FALSE(size == 8 * page_size);
> > +	FAIL_TEST_IF_FALSE(prot == 0x4);
> > +
> > +	if (seal) {
> > +		ret = sys_mseal(ptr + 4 * page_size, 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(!ret);
> > +	}
> > +
> > +	/* use mmap to expand and overwrite (MAP_FIXED)  */
>
> You don't really need to say MAP_FIXED, it's below.
>
> > +	ret2 = mmap(ptr, 12 * page_size, PROT_READ,
> > +			MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
>
> Why read-only?
>
> You're not expanding you're overwriting. You're not doing anything in the
> middle.
>
> I'm again confused about what you think you're testing here. I don't think
> we need an arbitrary MAP_FIXED mmap() at a size larger than the overwritten
> VMA?
>
> You just need a single instance of a MAP_FIXED mmap() over a sealed mmap()
> if that's what you want.
>
> > +	if (seal) {
> > +		FAIL_TEST_IF_FALSE(ret2 == MAP_FAILED);
> > +		FAIL_TEST_IF_FALSE(errno == EPERM);
> > +
> > +		size = get_vma_size(ptr, &prot);
> > +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(prot == 0x4);
> > +
> > +		size = get_vma_size(ptr + 4 * page_size, &prot);
> > +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(prot == 0x4);
> > +	} else
> > +		FAIL_TEST_IF_FALSE(ret2 == ptr);
>
> Don't do dangling else's after a big block.
>
> > +
> > +	REPORT_TEST_PASS();
> > +}
> > +
> > +static void test_seal_mmap_shrink_seal_middle(bool seal)
>
> What's going on in the 'middle'? This test doesn't shrink, it overwrites
> the beginning of a sealed VMA?

Correction - the middle is sealed. Other points remain.

> > +{
> > +	void *ptr;
> > +	unsigned long page_size = getpagesize();
> > +	unsigned long size = 12 * page_size;
> > +	int ret;
> > +	void *ret2;
> > +	int prot;
> > +
> > +	setup_single_address(size, &ptr);
> > +	FAIL_TEST_IF_FALSE(ptr != (void *)-1);
> > +
> > +	if (seal) {
> > +		ret = sys_mseal(ptr + 4 * page_size, 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(!ret);
> > +	}
> > +
> > +	/* use mmap to shrink and overwrite (MAP_FIXED)  */
>
> What exactly are you shrinking? You're overwriting the start of the vma?
>
> What is this testing that is different from the previous test? This seems
> useless honestly.
>
> > +	ret2 = mmap(ptr, 7 * page_size, PROT_READ,
> > +			MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
> > +	if (seal) {
> > +		FAIL_TEST_IF_FALSE(ret2 == MAP_FAILED);
> > +		FAIL_TEST_IF_FALSE(errno == EPERM);
> > +
> > +		size = get_vma_size(ptr, &prot);
> > +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(prot == 0x4);
>
> What the hell is this comparison to magic numbers? This is
> ridiculous. What's wrong with PROT_xxx??
>
> > +
> > +		size = get_vma_size(ptr + 4 * page_size, &prot);
> > +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(prot == 0x4);
> > +
> > +		size = get_vma_size(ptr + 4 * page_size, &prot);
> > +		FAIL_TEST_IF_FALSE(size == 4 * page_size);
> > +		FAIL_TEST_IF_FALSE(prot == 0x4);
>
> Err dude, you're doing this twice?
>
> So what are we testing here exactly? That we got a VMA split? This is
> err... why are we asserting this?

I guess, that we can't overwrite a sealed bit of a VMA at the end. But again
this feels entirely redundant. For this kind of thing to fail would mean the
whole VMA machinery is broken.

>
> > +	} else
> > +		FAIL_TEST_IF_FALSE(ret2 == ptr);
> > +
> > +	REPORT_TEST_PASS();
> > +}
> > +
> > +static void test_seal_mmap_reuse_addr(bool seal)
>
> This is wrong, you're not reusing anything. This test is useless.
>
> > +{
> > +	void *ptr;
> > +	unsigned long page_size = getpagesize();
> > +	unsigned long size = page_size;
> > +	int ret;
> > +	void *ret2;
> > +	int prot;
> > +
> > +	setup_single_address(size, &ptr);
> > +	FAIL_TEST_IF_FALSE(ptr != (void *)-1);
> > +
> > +	if (seal) {
> > +		ret = sys_mseal(ptr, size);
> > +		FAIL_TEST_IF_FALSE(!ret);
>
> We could avoid this horrid ret, ret2 naming if you just did:
>
> 	FAIL_TEST_IF_FALSE(sys_mseal(ptr, size));
>
> > +	}
> > +
> > +	/* use mmap to change protection. */
> > +	ret2 = mmap(ptr, size, PROT_NONE,
> > +			MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>
> How are you using mmap to change the protection when you're providing a
> hint to the address to use? You're not changing any protection at all!
>
> You're allocating an entirely new VMA hinting that you want it near
> ptr. Please read the man page for mmap():
>
>        If addr is NULL, then the kernel chooses the (page-aligned) address
>        at which to create the mapping; this is the most portable method of
>        creating a new mapping.  If addr is not NULL, then the kernel takes
>        it as a hint about where to place the mapping; on Linux, the kernel
>        will pick a nearby page boundary (but always above or equal to the
>        value specified by /proc/sys/vm/mmap_min_addr) and attempt to create
>        the mapping there.  If another mapping already exists there, the
>        kernel picks a new address that may or may not depend on the hint.
>        The address of the new mapping is returned as the result of the
>        call.
>
> > +
> > +	/* MAP_FIXED is not used, expect new addr */
> > +	FAIL_TEST_IF_FALSE(!(ret2 == MAP_FAILED));
>
> This is beyond horrible. You really have to add more asserts.
>
> Also you're expecting a new address here, so again, what on earth are you
> asserting? That we can mmap()?
>
> > +	FAIL_TEST_IF_FALSE(ret2 != ptr);
> > +
> > +	size = get_vma_size(ptr, &prot);
> > +	FAIL_TEST_IF_FALSE(size == page_size);
> > +	FAIL_TEST_IF_FALSE(prot == 0x4);
> > +
> > +	REPORT_TEST_PASS();
> > +}
> > +
> >  int main(int argc, char **argv)
> >  {
> >  	bool test_seal = seal_support();
> > @@ -2243,7 +2360,7 @@ int main(int argc, char **argv)
> >  	if (!get_vma_size_supported())
> >  		ksft_exit_skip("get_vma_size not supported\n");
> >
> > -	ksft_set_plan(91);
> > +	ksft_set_plan(97);
>
> I'm guessing this is the number of tests, but I mean this is horrible. Is
> there not a better way of doing this?
>
> >
> >  	test_seal_addseal();
> >  	test_seal_unmapped_start();
> > @@ -2357,5 +2474,12 @@ int main(int argc, char **argv)
> >  	test_munmap_free_multiple_ranges(false);
> >  	test_munmap_free_multiple_ranges(true);
> >
> > +	test_seal_mmap_expand_seal_middle(false);
> > +	test_seal_mmap_expand_seal_middle(true);
> > +	test_seal_mmap_shrink_seal_middle(false);
> > +	test_seal_mmap_shrink_seal_middle(true);
> > +	test_seal_mmap_reuse_addr(false);
> > +	test_seal_mmap_reuse_addr(true);
> > +
> >  	ksft_finished();
> >  }
> > --
> > 2.46.0.469.g59c65b2a67-goog
> >

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