[MODERATED] Re: [patch V6 07/14] MDS basics 7

On Tue, Mar 05, 2019 at 04:30:38PM +0100, speck for Thomas Gleixner wrote:
> On Sat, 2 Mar 2019, speck for Frederic Weisbecker wrote:
> 
> > On Fri, Mar 01, 2019 at 10:47:45PM +0100, speck for Thomas Gleixner wrote:
> > > +
> > > +   - Debug Exception (#DB):
> > > +
> > > +     This takes the paranoid exit path only when the INT1 breakpoint is in
> > > +     kernel space. #DB on a user space address takes the regular exit path,
> > > +     so no extra mitigation required.
> > 
> > I can't find that part in this patch, maybe it's further in the series?
> 
> There is no patch. #DB is not interesting as explained above.

Oh right, my brainfart...

> 
> > > --- a/arch/x86/kernel/nmi.c
> > > +++ b/arch/x86/kernel/nmi.c
> > > @@ -34,6 +34,7 @@
> > >  #include <asm/x86_init.h>
> > >  #include <asm/reboot.h>
> > >  #include <asm/cache.h>
> > > +#include <asm/nospec-branch.h>
> > >  
> > >  #define CREATE_TRACE_POINTS
> > >  #include <trace/events/nmi.h>
> > > @@ -533,6 +534,9 @@ do_nmi(struct pt_regs *regs, long error_
> > >  		write_cr2(this_cpu_read(nmi_cr2));
> > >  	if (this_cpu_dec_return(nmi_state))
> > >  		goto nmi_restart;
> > > +
> > > +	if (user_mode(regs))
> > > +		mds_user_clear_cpu_buffers();
> > 
> > What if the NMI fires after a call to prepare_exit_to_usermode()
> > but before the actual return to usermode, would that be a problem?
> 
> Yes, it's a hole in the protection, but you would need to be able to
> orchestrate that as user which I doubt you can. So the thought was that we
> rather avoid the penalty for perf when it hits kernel space, which requires
> root ....

Fair enough.

Reviewed-by: Frederic Weisbecker <[email protected]>