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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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/