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

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?

> --- 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?

Thanks.