Re: [GIT PULL] x86/topology changes for v5.3

[Peter Zijlstra]

On Wed, Jul 10, 2019 at 04:22:51PM +0200, Jiri Kosina wrote:
> On Wed, 10 Jul 2019, Peter Zijlstra wrote:
> 
> > If we mark the key as RO after init, and then try and modify the key to
> > link module usage sites, things might go bang as described.
> > 
> > Thanks!
> > 
> > 
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 27d7864e7252..5bf7a8354da2 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -366,7 +366,7 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
> >  	cr4_clear_bits(X86_CR4_UMIP);
> >  }
> >  
> > -DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
> > +DEFINE_STATIC_KEY_FALSE(cr_pinning);
> 
> Good catch, I guess that is going to fix it.
> 
> At the same time though, it sort of destroys the original intent of Kees' 
> patch, right? The exploits will just have to call static_key_disable() 
> prior to calling native_write_cr4() again, and the protection is gone.

This is fixable by moving native_write_cr*() out-of-line, such that they
never end up in a module.