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

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Date:   Sun, 22 Oct 2023 18:16:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kvm@...r.kernel.org,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com,
        Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

On Sat, Oct 21, 2023 at 12:50:37AM +0100, Andrew Cooper wrote:
> On 20/10/2023 9:44 pm, 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;			\
> > +
> 
> Is this actually wise from a perf point of view?
> 
> You're causing a data access to the instruction stream, and not only
> that, the immediate next instruction.  Some parts don't take kindly to
> snoops hitting L1I.
> 
> A better option would be to simply have
> 
> .section .text.entry
> .align CACHELINE
> mds_verw_sel:
>     .word __KERNEL_DS
>     int3
> .align CACHELINE
> 
> 
> And then just have EXEC_VERW be
> 
>     verw mds_verw_sel(%rip)

	ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_USER_CLEAR_CPU_BUF

But yeah, his seems like the sanest form.