Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

[Peter Zijlstra]

On Wed, Feb 14, 2018 at 09:56:14AM +0100, Peter Zijlstra wrote:
> On Tue, Feb 13, 2018 at 05:49:47PM -0800, Tim Chen wrote:
> 
> >  static inline void firmware_restrict_branch_speculation_start(void)
> >  {
> > +	if (this_cpu_inc_return(spec_ctrl_ibrs_fw_depth) == 1)
> > +		alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS,
> >  			      X86_FEATURE_USE_IBRS_FW);
> >  }
> >  
> >  static inline void firmware_restrict_branch_speculation_end(void)
> >  {
> > +	if (this_cpu_dec_return(spec_ctrl_ibrs_fw_depth) == 0)
> > +		alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
> > +				      X86_FEATURE_USE_IBRS_FW);
> >  }
> 
> 
> At the very least this must disable and re-enable preemption, such that
> we guarantee we inc/dec the same counter. ISTR some firmware calls (EFI)
> actually are preemptible so that wouldn't work.
> 
> Further, consider:
> 
> 	this_cpu_inc_return()		// 0->1
> 	<NMI>
> 		this_cpu_inc_return()	// 1->2
> 		call_broken_arse_firmware()
> 		this_cpu_dec_return()	// 2->1
> 	</NMI>
> 	wrmsr(SPEC_CTRL, IBRS);
> 
> 	/* from dodgy firmware crap */

s/from/more/

typing hard.

> 	this_cpu_dec_return()		// 1->0
> 	wrmsr(SPEC_CTRL, 0);