linux-kernel - Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

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Date:   Sun, 21 Jan 2018 20:28:17 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        KarimAllah Ahmed <karahmed@...zon.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        x86@...nel.org, Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation

On Sun, 2018-01-21 at 11:34 -0800, Linus Torvalds wrote:
> All of this is pure garbage.
> 
> Is Intel really planning on making this shit architectural? Has
> anybody talked to them and told them they are f*cking insane?
> 
> Please, any Intel engineers here - talk to your managers. 

If the alternative was a two-decade product recall and giving everyone
free CPUs, I'm not sure it was entirely insane.

Certainly it's a nasty hack, but hey — the world was on fire and in the
end we didn't have to just turn the datacentres off and go back to goat
farming, so it's not all bad.

As a hack for existing CPUs, it's just about tolerable — as long as it
can die entirely by the next generation.

So the part is I think is odd is the IBRS_ALL feature, where a future
CPU will advertise "I am able to be not broken" and then you have to
set the IBRS bit once at boot time to *ask* it not to be broken. That
part is weird, because it ought to have been treated like the RDCL_NO
bit — just "you don't have to worry any more, it got better".

https://software.intel.com/sites/default/files/managed/c5/63/336996-Speculative-Execution-Side-Channel-Mitigations.pdf

We do need the IBPB feature to complete the protection that retpoline
gives us — it's that or rebuild all of userspace with retpoline.

We'll also want to expose IBRS to VM guests, since Windows uses it.

I think we could probably live without the IBRS frobbing in our own
syscall/interrupt paths, as long as we're prepared to live with the
very hypothetical holes that still exist on Skylake. Because I like
IBRS more... no, let me rephrase... I hate IBRS less than I hate the
'deepstack' and other stuff that was being proposed to make Skylake
almost safe with retpoline.
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