From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755783AbeAIN6z (ORCPT + 1 other); Tue, 9 Jan 2018 08:58:55 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:60330 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752404AbeAIN6y (ORCPT ); Tue, 9 Jan 2018 08:58:54 -0500 Message-ID: <1515506318.22302.46.camel@infradead.org> Subject: Re: [PATCH] x86/retpoline: Also fill return buffer after idle From: David Woodhouse To: Peter Zijlstra , Andi Kleen Cc: pjt@google.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, gregkh@linux-foundation.org, tim.c.chen@linux.intel.com, dave.hansen@intel.com, tglx@linutronix.de, luto@amacapital.net, Andi Kleen In-Reply-To: <20180109093756.GF6176@hirez.programming.kicks-ass.net> References: <20180108235126.28736-1-andi@firstfloor.org> <20180109093756.GF6176@hirez.programming.kicks-ass.net> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-jZj7ab2CpnNR8kns4Zb9" Date: Tue, 09 Jan 2018 13:58:38 +0000 Mime-Version: 1.0 X-Mailer: Evolution 3.18.5.2-0ubuntu3.2 X-SRS-Rewrite: SMTP reverse-path rewritten from by twosheds.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: --=-jZj7ab2CpnNR8kns4Zb9 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2018-01-09 at 10:37 +0100, Peter Zijlstra wrote: > On Mon, Jan 08, 2018 at 03:51:26PM -0800, Andi Kleen wrote: >=20 > >=20 > > @@ -107,8 +109,15 @@ static inline void mwait_idle_with_hints(unsigned = long eax, unsigned long ecx) > > =C2=A0 } > > =C2=A0 > > =C2=A0 __monitor((void *)=C2=A4t_thread_info()->flags, 0, 0); > > - if (!need_resched()) > > + if (!need_resched()) { > > =C2=A0 __mwait(eax, ecx); > > + /* > > + =C2=A0* idle could have cleared the return buffer, > > + =C2=A0* so fill it to prevent uncontrolled > > + =C2=A0* speculation. > > + =C2=A0*/ > > + fill_return_buffer(); > wouldn't something like: >=20 > if (static_cpu_has(X86_FEATURE_RETPOLINE)) > fill_return_buffer(); >=20 > be much saner? Then we avoid the entire call when not needed and you > don't have to muck with the asm either. Hm... The background, of course, that that we need to be careful when doing things like this. If you end up with a conditional branch there, then processor can speculate right past it. There's a reason a lot of the IBRS-setting code has, effectively, an 'else lfence' in the cases where it isn't being done with ALTERNATIVEs. We had a *beautiful* case of that in the early IBRS patch set, on the syscall path, where the conditional branch opened up a path for speculative execution all the way to the jmp *sys_call_table(=E2=80=A6). Now, as discussed on IRC, we can see that the current implementation of static_cpu_has using asm goto *is* generally doing the right thing and turning it into a straight unconditional jump over the fill_return_buffer() code. Clever GCC, have biscuit. However, you are suggesting that we turn the static_cpu_has() trick from a "nice to have" optimisation which is all very well when it pans out, to something we *rely* on for secure operation of the system. It never ends well when we rely on all versions of GCC optimising things precisely how we want. If you can build in a sanity check to ensure that the build will *fail* when GCC doesn't do what we want, I suppose we could live with that. 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