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

From: Dave Hansen <dave.hansen@intel.com>
To: Jann Horn <jannh@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-crypto@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Roxana Bradescu <roxabee@chromium.org>,
	Adam Langley <agl@google.com>, Ard Biesheuvel <ardb@kernel.org>,
	"Jason A . Donenfeld" <Jason@zx2c4.com>
Subject: Re: [PATCH] x86: enable Data Operand Independent Timing Mode
Date: Thu, 26 Jan 2023 08:40:29 -0800	[thread overview]
Message-ID: <394c92e2-a9aa-37e1-7a34-d7569ac844fd@intel.com> (raw)
In-Reply-To: <CAG48ez1NaWarARJj5SBdKKTYFO2MbX7xO75Rk0Q2iK8LX4BwFA@mail.gmail.com>

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.