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

Pawan Gupta
2023-10-21T01:01:17Z

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.