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

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Date:	Thu, 15 May 2014 23:53:20 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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 12:46:34PM -0700, Andy Lutomirski wrote:
> 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.

As far as I see _install_special_mapping allocates one vma from cache and

	vma->vm_start = addr;
	vma->vm_end = addr + len;

so where is the second one?

> 
> >
> > [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?

x86-64, 3.15-rc5

> 
> > 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 :)

Good to know ;)

> 
> >
> >  - 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 :)

Yeah, I know, we simply had no choise.
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