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

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Message-ID: <CALCETrWbf8XYvBh=zdyOBqVqRd7s8SVbbDX=O2X+zAZn83r-bw@mail.gmail.com>
Date:	Thu, 15 May 2014 12:59:04 -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 12:53 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> 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?

Look at its callers in vdso32-setup.c and/or vma.c, depending on version.

>
>>
>> >
>> > [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

Aha.  Give tip/x86/vdso or -next a try or boot a 32-bit 3.15-rc kernel
and you'll see it.

--Andy
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