linux-kernel - Re: [PATCH v7 1/6] x86/bugs: Add asm helpers for executing VERW

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Date: Fri, 9 Feb 2024 18:28:43 +0100
From: Borislav Petkov <bp@...en8.de>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Nikolay Borisov <nik.borisov@...e.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>, stable@...nel.org
Subject: Re: [PATCH  v7 1/6] x86/bugs: Add asm helpers for executing VERW

On Sun, Feb 04, 2024 at 11:18:59PM -0800, Pawan Gupta wrote:
>  .popsection
> +
> +/*
> + * Defines the VERW operand that is disguised as entry code so that

"Define..."

> + * it can be referenced with KPTI enabled. This ensures VERW can be

"Ensure..."

But committer can fix those.

> + * used late in exit-to-user path after page tables are switched.
> + */
> +.pushsection .entry.text, "ax"
> +
> +.align L1_CACHE_BYTES, 0xcc
> +SYM_CODE_START_NOALIGN(mds_verw_sel)
> +	UNWIND_HINT_UNDEFINED
> +	ANNOTATE_NOENDBR
> +	.word __KERNEL_DS
> +.align L1_CACHE_BYTES, 0xcc
> +SYM_CODE_END(mds_verw_sel);
> +/* For KVM */
> +EXPORT_SYMBOL_GPL(mds_verw_sel);
> +
> +.popsection
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index fdf723b6f6d0..2b62cdd8dd12 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -95,7 +95,7 @@
>  #define X86_FEATURE_SYSENTER32		( 3*32+15) /* "" sysenter in IA32 userspace */
>  #define X86_FEATURE_REP_GOOD		( 3*32+16) /* REP microcode works well */
>  #define X86_FEATURE_AMD_LBR_V2		( 3*32+17) /* AMD Last Branch Record Extension Version 2 */
> -/* FREE, was #define X86_FEATURE_LFENCE_RDTSC		( 3*32+18) "" LFENCE synchronizes RDTSC */
> +#define X86_FEATURE_CLEAR_CPU_BUF	( 3*32+18) /* "" Clear CPU buffers using VERW */
>  #define X86_FEATURE_ACC_POWER		( 3*32+19) /* AMD Accumulated Power Mechanism */
>  #define X86_FEATURE_NOPL		( 3*32+20) /* The NOPL (0F 1F) instructions */
>  #define X86_FEATURE_ALWAYS		( 3*32+21) /* "" Always-present feature */
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 262e65539f83..ec85dfe67123 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -315,6 +315,21 @@
>  #endif
>  .endm
>  
> +/*
> + * Macros 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 EXEC_VERW

I think I asked this already:

Why isn't this called simply "VERW"?

There's no better name as this is basically the insn itself...

> +	verw _ASM_RIP(mds_verw_sel)
> +.endm

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette