Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user space protection mode

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Lutomirski <luto@kernel.org>,
	Jiri Kosina <jkosina@suse.cz>,
	thomas.lendacky@amd.com, Josh Poimboeuf <jpoimboe@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Andi Kleen <ak@linux.intel.com>,
	dave.hansen@intel.com,
	Casey Schaufler <casey.schaufler@intel.com>,
	"Mallick, Asit K" <asit.k.mallick@intel.com>,
	"Van De Ven, Arjan" <arjan@linux.intel.com>,
	jcm@redhat.com, longman9394@gmail.com,
	Greg KH <gregkh@linuxfoundation.org>,
	david.c.stewart@intel.com, Kees Cook <keescook@chromium.org>,
	jason.w.brandt@intel.com
Subject: Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user space protection mode
Date: Tue, 4 Dec 2018 09:20:52 -0800	[thread overview]
Message-ID: <CAHk-=wj7h=eCrvEPuO_AJ2Y5T5akTQPs5cLA811O64wORf+NyQ@mail.gmail.com> (raw)
In-Reply-To: <f4b903ef-828c-0b05-29c6-6f71677d120d@linux.intel.com>

On Mon, Dec 3, 2018 at 5:38 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> To make the usage of STIBP and its working principles clear,
> here are some additional explanations of STIBP from our Intel
> HW architects.  This should also help answer some of the questions
> from Thomas and others on STIBP's usages with IBPB and IBRS.
>
> Thanks.
>
> Tim
>
> ---
>
> STIBP
> ^^^^^
> Implementations of STIBP on existing Core-family processors (where STIBP
> functionality was added through a microcode update) work by disabling
> branch predictors that both:
>
>     1. Contain indirect branch predictions for both hardware threads, and
>     2. Do not contain a dedicated thread ID bit

Honestly, it still feels entirely misguided to me.

The above is not STIBP. It's just "disable IB". There's nothing "ST" about it.

So on processors where there is no thread ID bit (or per-thread
predictors), Intel simply SHOULD NOT EXPOSE this at all.

As it is, I refuse to call this shit "STIBP", because on current CPU's
that's simply a lie.

Being "technically correct" is not an excuse. It's just lying. I would
really hope that we restrict the lying to politicians, and not do it
in technical documentation.

               Linus