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

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Date:	Thu, 15 May 2014 12:46:34 -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 Thu, May 15, 2014 at 1:45 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, May 14, 2014 at 03:23:27PM -0700, Andy Lutomirski wrote:
>> 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.
>
> Andy, could you please point me where is the code which creates a second vma?
> latest 3.15 master branch

Search for _install_special_mapping in arch/x86/vdso.  It's in a
different place in 3.15-rc and -next.

>
> [root@fc ~]# cat /proc/self/maps
> ...
> 7fff57b6e000-7fff57b8f000 rw-p 00000000 00:00 0                          [stack]
> 7fff57bff000-7fff57c00000 r-xp 00000000 00:00 0                          [vdso]
> ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
> [root@fc ~]#
>

What version and bitness is this?

> Or you mean vsyscall area? If yes, then in criu we don't dump vsyscall zone.
> On restore we don't touch  vsyscall either but for vdso there are two cases

vsyscalls are almost gone now :)

>
>  - if there were no kernel change on vdso contents we simply use vdso provided
>    by the kernel at the moment of criu startup
>
>  - if vdso has been changed and looks different from one saved in image during
>    checkpoint, we map it from image but then patch (push jmp instruction) so
>    when application calls for some of vdso function it jumps into vdso code
>    saved in image and then jumps into vdso mapped by the kernel (ie kind of
>    proxy calls) This force us to do own Elf parsing inside criu to calculate
>    proper offsets.

Yuck :)

--Andy
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