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

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Date:   Mon, 15 Jan 2018 06:42:03 -0800
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
        "riel@...hat.com" <riel@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/pti] x86/retpoline: Fill RSB on context switch for
 affected CPUs

> 
> This would means that userspace would see return predictions based
> on the values the kernel 'stuffed' into the RSB to fill it.
> 
> Potentially this leaks a kernel address to userspace.

KASLR pretty much died in May this year to be honest with the KAISER paper (if not before then)

also with KPTI the address won't have a TLB mapping so it wouldn't
actually be speculated into.