Re: [PATCH v3 3/3] x86/asm: Pin sensitive CR0 bits

[Kees Cook <keescook@chromium.org>]

On Tue, Jun 18, 2019 at 02:24:30PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 18, 2019 at 11:38:02AM +0200, Jann Horn wrote:
> > On Tue, Jun 18, 2019 at 6:55 AM Kees Cook <keescook@chromium.org> wrote:
> > > With sensitive CR4 bits pinned now, it's possible that the WP bit for
> > > CR0 might become a target as well. Following the same reasoning for
> > > the CR4 pinning, this pins CR0's WP bit (but this can be done with a
> > > static value).
> > >
> > > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > ---
> > >  arch/x86/include/asm/special_insns.h | 15 ++++++++++++++-
> > >  1 file changed, 14 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> > > index c8c8143ab27b..b2e84d113f2a 100644
> > > --- a/arch/x86/include/asm/special_insns.h
> > > +++ b/arch/x86/include/asm/special_insns.h
> > > @@ -31,7 +31,20 @@ static inline unsigned long native_read_cr0(void)
> > >
> > >  static inline void native_write_cr0(unsigned long val)
> > >  {
> > 
> > So, assuming a legitimate call to native_write_cr0(), we come in here...
> > 
> > > -       asm volatile("mov %0,%%cr0": : "r" (val), "m" (__force_order));
> > > +       unsigned long bits_missing = 0;
> 
> ^^^
> 
> > > +
> > > +set_register:
> > > +       asm volatile("mov %0,%%cr0": "+r" (val), "+m" (__force_order));
> > 
> > ... here we've updated CR0...
> > 
> > > +       if (static_branch_likely(&cr_pinning)) {
> > 
> > ... this branch is taken, since cr_pinning is set to true after boot...
> > 
> > > +               if (unlikely((val & X86_CR0_WP) != X86_CR0_WP)) {
> > 
> > ... this branch isn't taken, because a legitimate update preserves the WP bit...
> > 
> > > +                       bits_missing = X86_CR0_WP;
> 
> ^^^
> 
> > > +                       val |= bits_missing;
> > > +                       goto set_register;
> > > +               }
> > > +               /* Warn after we've set the missing bits. */
> > > +               WARN_ONCE(bits_missing, "CR0 WP bit went missing!?\n");
> > 
> > ... and we reach this WARN_ONCE()? Am I missing something, or does
> > every legitimate CR0 write after early boot now trigger a warning?
> 
> bits_missing will be 0 and WARN will not be issued.
> 
> > > +       }
> > >  }

Yup, as Peter points out, bits_missing is only non-zero when bits went
missing. The normal case will skip the WARN_ONCE() (which is also
internally wrapped in unlikely()). And I would have noticed the very
loud WARN at every boot if this wasn't true. ;)

-- 
Kees Cook