linux-kernel - Re: x86/clearregs: Register sanitizing at kernel entry for speculation hygiene

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Date:   Tue, 9 Jan 2018 17:39:04 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, dwmw@...zon.co.uk, pjt@...gle.com,
        luto@...nel.org, peterz@...radead.org, thomas.lendacky@....com,
        tim.c.chen@...ux.intel.com, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, jikos@...nel.org
Subject: Re: x86/clearregs: Register sanitizing at kernel entry for speculation hygiene



On Jan 9, 2018, at 5:34 PM, Andi Kleen <andi@...stfloor.org> wrote:

>> I don't like this at all.  Once upon a time, Linux syscalls were supposed to be fast.  Then we learned about the Meltdown screwup, so we mostly fixed it for real upstream and the distroa seriously half-arsed their own fixes [1].  This came with a big performance cost, but it can be turned off on non-busted hardware.  So be it.
> 
> That's true, but modern CPUs are also a lot faster/wider than the K8
> the fast path was originally designed for. A modern CPU can go through
> these instructions really fast with a very high IPC because they don't have
> dependencies or stalls.
> 
> So it shouldn't hurt very much.
> 
> Also in fact when the fast path was originally written the ABI still had a
> different caller/callee split which made it more better. Later on
> it already lost some of its benefits and was less of a win.
> 
>> But now we're proposing to throw out the whole fast path because it might make it a bit harder to do the most obvious attack.  Not very hard, mind you, but a little bit harder.  And there's no off switch for less-leaky hardware.  No thanks.
> 
> Well the off switch is a fast CPU.

When I rewrote the fast path, I did it on SNB.  Not much has changed.

This patch should come with benchmarks (with PTI off).

And Intel needs to come up with real fixes for this stuff.