Re: [x86, vdso] BUG: unable to handle kernel paging request at d34bd000

[Stefani Seibold <stefani@seibold.net>]

Am Montag, den 10.03.2014, 10:12 -0700 schrieb Andy Lutomirski:
> On Mon, Mar 10, 2014 at 8:11 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Mar 10, 2014 8:01 AM, "H. Peter Anvin" <hpa@linux.intel.com> wrote:
> >>
> >> I have mentioned in the past wanting to move the fixmap to the low part
> >> of the kernel space, because the top isn't really fixed...
> >
> > How about the high part of the user address space, just above the stack?
> > Leave a unmapped page in between, or something. The stack is already
> > randomized, isn't it?
> 
> For the !compat_vdso case, I don't like it -- this will put the vdso
> (which is executable) at a constant offset from the stack, which will
> make it much easier to use the vdso to defeat ASLR.
> 
> For the compat_vdso case, this only works if the address is *not*
> random, unless we're going to start giving each process its very own
> relocated vdso.
> 
> >
> > That would actually be preferable in a few ways, notably not having to mark
> > page directories user accessible in the kennel space area.
> 
> Is that where the rabid pte dogs live?
> 
> We can already avoid making fixmap pages user-accessible in the
> !compat_vdso case for 32-bit tasks -- the vdso lives in a couple of
> more-or-less ordinary vmas.
> 

What is now the next step? Kick out the compat VDSO? Or should i
implement the dual VDSO. And what is now the preferred way to map the
VDSO into the user space? Using install_special_mapping() or map it
beyond the user stack?

The is easiest and fastest way to get a working result is to do the non
compat VDSO only mapping using install_special_mapping(). The dual VDSO
would take a little bit more time.

It would be great to have first a consensus about the design before i
start to implement ;-)

- Stefani