Re: [tip: x86/urgent] x86/bugs: Add asm helpers for executing VERW

[Nikolay Borisov]

On 27.02.24 г. 0:10 ч., Pawan Gupta wrote:
> On Mon, Feb 26, 2024 at 09:17:30AM +0200, Nikolay Borisov wrote:
> > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > > index 262e655..077083e 100644
> > > --- a/arch/x86/include/asm/nospec-branch.h
> > > +++ b/arch/x86/include/asm/nospec-branch.h
> > > @@ -315,6 +315,17 @@
> > >    #endif
> > >    .endm
> > > +/*
> > > + * Macro to execute VERW instruction that mitigate transient data sampling
> > > + * attacks such as MDS. On affected systems a microcode update overloaded VERW
> > > + * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
> > > + *
> > > + * Note: Only the memory operand variant of VERW clears the CPU buffers.
> > > + */
> > > +.macro CLEAR_CPU_BUFFERS
> > > +	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> >
> > Any particular reason why this uses RIP-relative vs an absolute address
> > mode?
>
> Early versions of the series had the VERW arg pointing to the macro
> itself, that is why relative addressing was used. That got changed in a
> later version with all VERW sites pointing to a single memory location.
>
> > I know in our private exchange you said there is no significance but
> > for example older kernels have a missing relocation support in alternatives.
> > This of course can be worked around by slightly changing the logic of the
> > macro which means different kernels will have slightly different macros.
>
> Do you anticipate a problem with that? If yes, I can send a patch to use
> fixed addressing in upstream as well.

I experienced crashes on older kernels before realizing that the 
relocation wasn't resolved correctly by the alternative framework. 
Instead i simply changed the macro to jmp 1f, where the next instruction 
is the verw ( I did send a backport for 5.4) and it works. Recently 
there's been a push to make as much of the kernel assembly as possible 
PIC so having a rip-relative addressing helps. Whether that makes any 
material difference - I cannot say.

Here's my backport version for reference:

https://lore.kernel.org/stable/20240226122237.198921-3-nik.borisov@xxxxxxxx/