Re: [patch 11/16] x86/ldt: Force access bit for CS/SS

[Peter Zijlstra]

On Tue, Dec 12, 2017 at 10:03:02AM -0800, Andy Lutomirski wrote:
> On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> > @@ -171,6 +172,9 @@ static void exit_to_usermode_loop(struct
> >                 /* Disable IRQs and retry */
> >                 local_irq_disable();
> >
> > +               if (cached_flags & _TIF_LDT)
> > +                       ldt_exit_user(regs);
> 
> Nope.  To the extent that this code actually does anything (which it
> shouldn't since you already forced the access bit),

Without this; even with the access bit set; IRET will go wobbly and
we'll #GP on the user-space side. Try it ;-)

> it's racy against
> flush_ldt() from another thread, and that race will be exploitable for
> privilege escalation.  It needs to be outside the loopy part.

The flush_ldt (__ldt_install after these patches) would re-set the TIF
flag. But sure, we can move this outside the loop I suppose.