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

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Date:   Thu, 26 Jan 2023 11:12:36 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Roxana Bradescu <roxabee@...omium.org>,
        Adam Langley <agl@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH] x86: enable Data Operand Independent Timing Mode

On 1/26/23 09:52, Jann Horn wrote:
>> 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 list says at the top: "The table below lists instructions that
> have data-independent timing."

So, first of all, apologies for the documentation.  It needs some work,
and I see where the confusion is coming from.

But, I did just confirm with the folks that wrote it. The "listed
instructions" *ARE* within the scope of being affected by the DOITM=0/1
setting.

Instead of saying:

	The table below lists instructions that have data-independent
	timing.

I think it should probably say something like:

	The table below lists instructions that have data-independent
	timing when DOITM is enabled.

	(Modulo the MXCSR interactions for now)

Somebody from Intel please thwack me over the head if I'm managing to
get this wrong (wouldn't be the first time).