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

[Pawan Gupta]

On Tue, Feb 27, 2024 at 12:20:03AM +0200, Nikolay Borisov wrote:
> 
> 
> 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.

Ok, sending the patch.

> Here's my backport version for reference:
> 
> https://lore.kernel.org/stable/20240226122237.198921-3-nik.borisov@xxxxxxxx/

Below should also solve the problem with less churn:

---
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 2aa52cab1e46..ab19c7f1167b 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -323,7 +323,7 @@
  * 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
+	ALTERNATIVE "", __stringify(verw mds_verw_sel), X86_FEATURE_CLEAR_CPU_BUF
 .endm
 
 #else /* __ASSEMBLY__ */