linux-kernel - Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW operand

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Message-ID: <efa42b69-13ba-40ed-99e2-431220d7dcb3@citrix.com>
Date: Wed, 9 Oct 2024 11:24:19 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
 Robert Gill <rtgill82@...il.com>, stable@...r.kernel.org,
 #@...-bot2.tec.linutronix.de, 5.10+@...-bot2.tec.linutronix.de,
 Dave Hansen <dave.hansen@...ux.intel.com>, Brian Gerst <brgerst@...il.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org
Subject: Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW
 operand

On 09/10/2024 10:32 am, Borislav Petkov wrote:
> On Wed, Oct 09, 2024 at 09:34:37AM +0200, Peter Zijlstra wrote:
>> You need ifdeffery either way around, either directly like this or for
>> that macro. This is simple and straight forward.
> Nothing in this file full of macros is simple. In any case, I would've done
> this as the ifdeffery is shorter and the macro is simpler. We have this coding
> pattern in a lot of headers, abstracting 32-bit vs 64-bit machine details, and
> it is a very common and familiar one:
>
> /*
>  * In 32bit mode, the memory operand must be a %cs reference. The data
>  * segments may not be usable (vm86 mode), and the stack segment may not be
>  * flat (ESPFIX32).
>  */
> #ifdef CONFIG_X86_64
> #define VERW_ARG "verw mds_verw_sel(%rip)"
> #else /* CONFIG_X86_32 */
> #define VERW_ARG "verw %cs:mds_verw_sel"
> #endif
>
> /*
>  * 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 "", VERW_ARG, X86_FEATURE_CLEAR_CPU_BUF
> .endm
>

I'll bite.  Why do you think this form is is better?

You've now got VERW_ARG leaking across the whole kernel, and a layer of
obfuscatio^W indirection in CLEAR_CPU_BUFFERS.

Admittedly, when I wrote this fragment as a suggestion[1], the 32bit
comment was in the main comment because there really is no need for it
to be separate.

But abstracting away VERW_ARG like this hampers legibility/clarity,
rather than improving it IMO.

~Andrew

[1]
https://lore.kernel.org/lkml/5703f2d8-7ca0-4f01-a954-c0eb1de930b4@citrix.com/