Re: [PATCH] x86: enable Data Operand Independent Timing Mode

Dave Hansen
2023-01-26T16:40:33Z

On 1/26/23 05:52, Jann Horn wrote:
> On Wed, Jan 25, 2023 at 4:30 PM Dave Hansen <dave.hansen@intel.com> wrote:
>> Translating from Intel-speak: Intel thinks that DOITM purely a way to
>> make the CPU run slower if you haven't already written code specifically
>> to mitigate timing side channels. All pain, no gain.
>>
>> The kernel as a whole is not written that way.
>
> The kernel as a whole also doesn't really use the FPU registers for
> anything other than checksumming and cryptography and stuff like that
> (it's disabled in the compiler flags because the FPU registers
> normally contain userspace state that must not be clobbered). The
> instructions listed on that Intel help page are all weird PM* and VP*
> arithmetic instructions that can't be generated from C code in the
> kernel (except for weird subsystems in which every function is only
> callable in kernel-FPU-enabled mode and the compiler is set to enable
> FPU instruction generation, by which I mean amdgpu).

Maybe I'm totally missing something, but I thought the scope here was
the "non-data operand independent timing behavior for the listed
instructions" referenced here:

> https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/data-operand-independent-timing-isa-guidance.html

where the "listed instructions" is this list:

> https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/resources/data-operand-independent-timing-instructions.html

For example, that includes XOR with the 0x31 and 0x81 opcodes which
there are plenty of in the kernel.

That's a bit wider scope than the crazy instructions like VPLZCNTD. The
crazy instructions list that I _think_ you were grepping for is the
"Instructions That May Exhibit MCDT Behavior". That's also a fun one,
but it is more narrow than the DOITM list.