linux-kernel - Re: [PATCH v2 1/2] x86/bugs: Don't fill RSB on VMEXIT with eIBRS+retpoline

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Message-ID: <g5xe26esmtoqevdgxueapvtvojgi63z3lsdzr3jyyo3cmcb2tj@gpeofgbzjzch>
Date: Wed, 2 Apr 2025 07:16:55 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: "Shah, Amit" <Amit.Shah@....com>
Cc: "bp@...en8.de" <bp@...en8.de>, "corbet@....net" <corbet@....net>, 
	"pawan.kumar.gupta@...ux.intel.com" <pawan.kumar.gupta@...ux.intel.com>, "kai.huang@...el.com" <kai.huang@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, 
	"daniel.sneddon@...ux.intel.com" <daniel.sneddon@...ux.intel.com>, "Lendacky, Thomas" <Thomas.Lendacky@....com>, 
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com" <mingo@...hat.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>, 
	"Moger, Babu" <Babu.Moger@....com>, "Das1, Sandipan" <Sandipan.Das@....com>, 
	"dwmw@...zon.co.uk" <dwmw@...zon.co.uk>, "hpa@...or.com" <hpa@...or.com>, 
	"peterz@...radead.org" <peterz@...radead.org>, "Kaplan, David" <David.Kaplan@....com>, 
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 1/2] x86/bugs: Don't fill RSB on VMEXIT with
 eIBRS+retpoline

On Wed, Apr 02, 2025 at 09:19:19AM +0000, Shah, Amit wrote:
> On Thu, 2024-12-05 at 15:12 -0800, Josh Poimboeuf wrote:
> > On Tue, Dec 03, 2024 at 12:20:15PM +0100, Borislav Petkov wrote:
> > > On Mon, Dec 02, 2024 at 03:35:21PM -0800, Pawan Gupta wrote:
> > > > It is in this doc:
> > > > 
> > > >  
> > > > https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/indirect-branch-restricted-speculation.html
> > > > 
> > > 
> > > I hope those URLs remain more stable than past experience shows.
> > > 
> > > >   "Processors with enhanced IBRS still support the usage model
> > > > where IBRS is
> > > >   set only in the OS/VMM for OSes that enable SMEP. To do this,
> > > > such
> > > >   processors will ensure that guest behavior cannot control the
> > > > RSB after a
> > > >   VM exit once IBRS is set, even if IBRS was not set at the time
> > > > of the VM
> > > >   exit."
> > > 
> > > ACK, thanks.
> > > 
> > > Now, can we pls add those excerpts to Documentation/ and point to
> > > them from
> > > the code so that it is crystal clear why it is ok?
> > 
> > Ok, I'll try to write up a document.  I'm thinking it should go in
> > its
> > own return-based-attacks.rst file rather than spectre.rst, which is
> > more
> > of an outdated historical document at this point.  And we want this
> > document to actually be read (and kept up to date) by developers
> > instead
> > of mostly ignored like the others.
> > 
> 
> Hey Josh,
> 
> Do you plan to submit a v3 with the changes?

Thanks for the reminder, I actually had the patches ready to go a few
months ago (with a fancy new doc) and then forgot to post.  Let me dust
off the cobwebs.

-- 
Josh