linux-kernel - Re: x86/retpoline: Fill RSB on context switch for affected CPUs

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Message-ID: <20180309151428.GE22087@tassilo.jf.intel.com>
Date:   Fri, 9 Mar 2018 07:14:28 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, thomas.lendacky@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: x86/retpoline: Fill RSB on context switch for affected CPUs

> Shouldn't the RSB filling on context switch also be done on non-IBPB
> CPUs to protect (retpolined) user space tasks from other user space
> tasks?

The comment is actually incorrect. There's no risk to hit user space
addresses if we have KPTI and NX (which is fairly universal).

It's mainly needed on Skylake era CPUs.

Should fix the comment. I'll send a patch.

-Andi