Re: [PATCH v3 1/6] x86/bugs: Add asm helpers for executing VERW

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

On 26/10/2023 2:44 pm, Nikolay Borisov wrote:
>
>
> <snip>
>> +
>> +.pushsection .entry.text, "ax"
>> +
>> +.align L1_CACHE_BYTES, 0xcc
>> +SYM_CODE_START_NOALIGN(mds_verw_sel)
>> +    UNWIND_HINT_UNDEFINED
>> +    ANNOTATE_NOENDBR
>> +    .word __KERNEL_DS
>> +SYM_CODE_END(mds_verw_sel);
>> +/* For KVM */
>> +EXPORT_SYMBOL_GPL(mds_verw_sel);
>> +
>> +.popsection
>
> <snip>
>
>> diff --git a/arch/x86/include/asm/nospec-branch.h
>> b/arch/x86/include/asm/nospec-branch.h
>> index c55cc243592e..005e69f93115 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -329,6 +329,21 @@
>>   #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
>> +    verw _ASM_RIP(mds_verw_sel)
>> +.endm
>> +
>> +.macro CLEAR_CPU_BUFFERS
>> +    ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF
>> +.endm
>
>
> What happened with the first 5 bytes of a 7 byte nop being
> complemented by __KERNEL_DS in order to handle VERW being executed
> after user registers are restored and having its memory operand ?

It was moved out of line (so no need to hide a constant in a nop),
deduped, and renamed to mds_verw_sel.

verw _ASM_RIP(mds_verw_sel) *is* the memory form.

~Andrew