Re: mm: NULL ptr deref handling mmaping of special mappings

Cyrill Gorcunov
2014-05-15T19:53:26Z

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@gmail.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.