linux-kernel - Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition

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Message-ID: <caeebbb1-f733-4969-8eab-ce69ce35f4c4@intel.com>
Date:   Mon, 23 Oct 2023 12:17:21 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.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
Subject: Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace
 transition

On 10/23/23 11:22, Josh Poimboeuf wrote:
> On Fri, Oct 20, 2023 at 01:45:03PM -0700, Pawan Gupta wrote:
>> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
>> +	USER_CLEAR_CPU_BUFFERS
>> +
>>  	jmp	.Lnative_iret
>>  
>>  
>> @@ -774,6 +780,9 @@ native_irq_return_ldt:
>>  	 */
>>  	popq	%rax				/* Restore user RAX */
>>  
>> +	/* Mitigate CPU data sampling attacks .e.g. MDS */
>> +	USER_CLEAR_CPU_BUFFERS
>> +
> 
> I'm thinking the comments add unnecessary noise here.  The macro name is
> self-documenting enough.
> 
> The detail about what mitigations are being done can go above the macro
> definition itself, which the reader can refer to if they want more
> detail about what the macro is doing and why.
> 
> Speaking of the macro name, I think just "CLEAR_CPU_BUFFERS" is
> sufficient.  The "USER_" prefix makes it harder to read IMO.

Yes, please.  The "USER_" prefix should be reserved for things that are
uniquely for the unambiguous return-to-userspace paths.