linux-kernel - Re: mm: NULL ptr deref handling mmaping of special mappings

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Date:	Wed, 14 May 2014 15:23:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: mm: NULL ptr deref handling mmaping of special mappings

On Wed, May 14, 2014 at 3:11 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, May 14, 2014 at 02:33:54PM -0700, Andy Lutomirski wrote:
>> On Wed, May 14, 2014 at 2:31 PM, Andrew Morton
>> <akpm@...ux-foundation.org> wrote:
>> > On Wed, 14 May 2014 17:11:00 -0400 Sasha Levin <sasha.levin@...cle.com> wrote:
>> >
>> >> > In my linux-next all that code got deleted by Andy's "x86, vdso:
>> >> > Reimplement vdso.so preparation in build-time C" anyway.  What kernel
>> >> > were you looking at?
>> >>
>> >> Deleted? It appears in today's -next. arch/x86/vdso/vma.c:124 .
>> >>
>> >> I don't see Andy's patch removing that code either.
>> >
>> > ah, OK, it got moved from arch/x86/vdso/vdso32-setup.c into
>> > arch/x86/vdso/vma.c.
>> >
>> > Maybe you managed to take a fault against the symbol area between the
>> > _install_special_mapping() and the remap_pfn_range() call, but mmap_sem
>> > should prevent that.
>> >
>> > Or the remap_pfn_range() call never happened.  Should map_vdso() be
>> > running _install_special_mapping() at all if
>> > image->sym_vvar_page==NULL?
>>
>> I'm confused: are we talking about 3.15-rcsomething or linux-next?
>> That code changed.
>>
>> Would this all make more sense if there were just a single vma in
>> here?  cc: Pavel and Cyrill, who might have to deal with this stuff in
>> CRIU
>
> Well, for criu we've not modified any vdso kernel's code (except
> setting VM_SOFTDIRTY for this vdso VMA in _install_special_mapping).
> And never experienced problems Sasha points. Looks like indeed in
> -next code is pretty different from mainline one. To figure out
> why I need to fetch -next branch and get some research. I would
> try to do that tomorrow (still hoping someone more experienced
> in mm system would beat me on that).

I can summarize:

On 3.14 and before, the vdso is just a bunch of ELF headers and
executable data.  When executed by 64-bit binaries, it reads from the
fixmap to do its thing.  That is, it reads from kernel addresses that
don't have vmas.  When executed by 32-bit binaries, it doesn't read
anything, since there was no 32-bit timing code.

On 3.15, the x86_64 vdso is unchanged.  The 32-bit vdso is preceded by
a separate vma containing two pages worth of time-varying read-only
data.  The vdso reads those pages using PIC references.

On linux-next, all vdsos work the same way.  There are two vmas.  The
first vma is executable text, which can be poked at by ptrace, etc
normally.  The second vma contains time-varying state, should not
allow poking, and is accessed by PIC references.

What does CRIU do to restore the vdso?  Will 3.15 and/or linux-next
need to make some concession for CRIU?

--Andy
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