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 17:16:02 -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

> If we clear the registers, what the hell are you going to put in the
> RSB that helps you?

RSB allows you to control chains of gadgets.

You can likely find some chain of gadgets that set up constants in registers in a
lot of useful ways. Perhaps not any way (so may be hard to scan through all of
memory), but it's likely you could find gadgets that result in a lot of useful
direct mapped addresses, which the next gadget can then reference.

Especially RAX is quite vulnerable to this because there will be a lot 
of code that does "modify RAX in interesting ways ; RET"

> So instead of saying "we have to flush the return stack", I'm saying
> that we should look at things that make flushing the return stack
> _unnecessary_, simply because even if the attacker were to control it
> entirely, they'd still be up shit creek without a paddle.

I agree that clearing registers is useful (was just hacking on that patch).

-Andi