linux-hardening - Re: [PATCH v1] selftest mm/mseal: fix test_seal_mremap_move_dontunmap

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Message-ID: <CALmYWFvDH=8U8wsaSjMrfCg1__S66SS-9Zo0f826XJDJT7hOSQ@mail.gmail.com>
Date: Wed, 7 Aug 2024 11:02:54 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: jeffxu@...omium.org, akpm@...ux-foundation.org, willy@...radead.org, 
	torvalds@...ux-foundation.org, Liam.Howlett@...cle.com, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-mm@...ck.org, dave.hansen@...el.com, linux-hardening@...r.kernel.org, 
	lorenzo.stoakes@...cle.com, mpe@...erman.id.au, oliver.sang@...el.com, 
	vbabka@...e.cz
Subject: Re: [PATCH v1] selftest mm/mseal: fix test_seal_mremap_move_dontunmap_anyaddr

On Wed, Aug 7, 2024 at 9:38 AM Pedro Falcato <pedro.falcato@...il.com> wrote:
>
> On Wed, Aug 7, 2024 at 4:35 PM <jeffxu@...omium.org> wrote:
> <snip>
> >         /* shrink from 4 pages to 2 pages. */
> > -       ret2 = mremap(ptr, size, 2 * page_size, 0, 0);
> > +       ret2 = sys_mremap(ptr, size, 2 * page_size, 0, 0);
> >         if (seal) {
> > -               FAIL_TEST_IF_FALSE(ret2 == MAP_FAILED);
> > +               FAIL_TEST_IF_FALSE(ret2 == (void *) MAP_FAILED);
>
> MAP_FAILED is already void *
>
> <snip>
> > @@ -1449,18 +1457,16 @@ static void test_seal_mremap_move_dontunmap_anyaddr(bool seal)
> >         }
> >
> >         /*
> > -        * The 0xdeaddead should not have effect on dest addr
> > +        * The 0xdead0000 should not have effect on dest addr
> >          * when MREMAP_DONTUNMAP is set.
> >          */
> > -       ret2 = mremap(ptr, size, size, MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
> > -                       0xdeaddead);
> > +       ret2 = sys_mremap(ptr, size, size, MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
> > +                       (void *) 0xdead0000);
>
> You still didn't explain why this test is actually needed. Why are you
> testing MREMAP_DONTUNMAP's hint system?

I responded in my previous email. The test is to make sure when
sealing is applied, the call fails with correct error code. I will
update the comment in v2 to clarify that.

> This has nothing to do with mseal, you already test the
> MREMAP_DONTUNMAP and MREMAP_FIXED paths in other tests.
The remap code path is quite tricky, with many flags directing the call flow.
The difference might not be that obvious:

test_seal_mremap_move_dontunmap use 0 as new_addr, 0 indicates
allocating a new memory.
test_seal_mremap_move_dontunmap_anyaddr uses any arbitrary address as
a new address.

> You also don't know if 0xdead0000 is a valid page (hexagon for
> instance seems to support 256KiB and 1MiB pages, so does ppc32, and
> this is not something that should be hardcoded).
>
usually hardcode value is not good practice, but the point of this
test is to show
mremap can really relocate the mapping to an arbitrary address.

Do you have any suggestions here ? I can think of two options to choose from:

1> use 0xd0000000
2> allocate a memory then free it, reuse the ptr.

Thanks
-Jeff