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

Dave Hansen
2023-10-23T19:17:27Z

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.