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

[Dave Hansen]

On 10/8/24 23:11, Borislav Petkov wrote:
>>  .macro CLEAR_CPU_BUFFERS
>> -	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
>> +#ifdef CONFIG_X86_64
>> +	ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
>> +#else
>> +	/*
>> +	 * 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).
>> +	 */
>> +	ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
>> +#endif
> So why didn't we ifdef the "verw mds_verw_sel(%rip)" and "verw
> %cs:mds_verw_sel" macro argument instead of adding more bigger ugly ifdeffery?

I'm not jumping for joy about how it looks, but I applied it because
it's good enough and the regression was about to get its driver's
license. ;)

I did spend some time noodling to come up with _some_ common 32/64-bit
implementation, but 32-bit is just too special of a snowflake.