linux-kernel - Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch

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Date:   Mon, 8 Jan 2018 16:44:15 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows
 on context switch

> So I was really hoping that in places like context switching etc, we'd
> be able to instead effectively kill off any exploits by clearing
> registers.
> 
> That should make it pretty damn hard to then find a matching "gadget"
> that actually does anything interesting/powerful.
> 
> Together with Spectre already being pretty hard to take advantage of,
> and the eBPF people making those user-proivided gadgets inaccessible,
> it really should be a pretty powerful fix.
> 
> Hmm?

Essentially the RSB are hidden registers, and the only way to clear them
is the FILL_RETURN_BUFFER sequence.  I don't see how clearing anything else
would help?

-Andi