Re: [PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Fri, Oct 20, 2023 at 04:13:13PM -0700, Sean Christopherson wrote:
> On Fri, Oct 20, 2023, Pawan Gupta wrote:
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index c55cc243592e..e1b623a27e1b 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -111,6 +111,24 @@
> >  #define RESET_CALL_DEPTH_FROM_CALL
> >  #endif
> >  
> > +/*
> > + * Macro to execute VERW instruction to mitigate transient data sampling
> > + * attacks such as MDS. On affected systems a microcode update overloaded VERW
> > + * instruction to also clear the CPU buffers.
> > + *
> > + * Note: Only the memory operand variant of VERW clears the CPU buffers. To
> > + * handle the case when VERW is executed after user registers are restored, use
> > + * RIP to point the memory operand to a part NOPL instruction that contains
> > + * __KERNEL_DS.
> > + */
> > +#define __EXEC_VERW(m)	verw _ASM_RIP(m)
> > +
> > +#define EXEC_VERW				\
> > +	__EXEC_VERW(551f);			\
> > +	/* nopl __KERNEL_DS(%rax) */		\
> > +	.byte 0x0f, 0x1f, 0x80, 0x00, 0x00;	\
> > +551:	.word __KERNEL_DS;			\
> 
> Why are there so many macro layers?  Nothing jumps out to justfying two layers,
> let alone three.

I can't remember the exact reason, but I think the reason I added
__EXEC_VERW() was because using EXEC_VERW() in a C function was leading
to build error in the internal draft version. This version is not
calling it from C, so yes I can get rid of the extra layer.

> >  /*
> >   * Fill the CPU return stack buffer.
> >   *
> > @@ -329,6 +347,13 @@
> >  #endif
> >  .endm
> >  
> > +/* Clear CPU buffers before returning to user */
> > +.macro USER_CLEAR_CPU_BUFFERS
> > +	ALTERNATIVE "jmp .Lskip_verw_\@;", "", X86_FEATURE_USER_CLEAR_CPU_BUF
> > +	EXEC_VERW
> 
> Rather than a NOP after VERW, why not something like this?
> 
> /* Clear CPU buffers before returning to user */
> .macro USER_CLEAR_CPU_BUFFERS
>                 ALTERNATIVE "jmp .Lskip_verw_\@;", "jmp .Ldo_verw_\@;", X86_FEATURE_USER_CLEAR_CPU_BUF
> 551:            .word __KERNEL_DS
> .Ldo_verw_\@:   verw _ASM_RIP(551b)
> .Lskip_verw_\@:
> .endm

I wasn't comfortable adding a variable directly in the instruction
stream because the CPU may interpret it wrongly. With NOP it is bound to
ignore the data part.