linux-kernel - RE: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch

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Message-ID: <859a318af2c9438ea134c57046b1712b@AcuMS.aculab.com>
Date:   Fri, 12 Jan 2018 11:15:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andi Kleen' <andi@...stfloor.org>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>
CC:     "pjt@...gle.com" <pjt@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "luto@...capital.net" <luto@...capital.net>,
        Andi Kleen <ak@...ux.intel.com>
Subject: RE: [PATCH] x86/retpoline: Avoid return buffer underflows on context
 switch

From: Andi Kleen
> Sent: 08 January 2018 20:16
>
> [This is on top of David's retpoline branch, as of 08-01 this morning]
> 
> This patch further hardens retpoline
> 
> CPUs have return buffers which store the return address for
> RET to predict function returns. Some CPUs (Skylake, some Broadwells)
> can fall back to indirect branch prediction on return buffer underflow.
> 
> With retpoline we want to avoid uncontrolled indirect branches,
> which could be poisoned by ring 3, so we need to avoid uncontrolled
> return buffer underflows in the kernel.
> 
> This can happen when we're context switching from a shallower to a
> deeper kernel stack.  The deeper kernel stack would eventually underflow
> the return buffer, which again would fall back to the indirect branch predictor.
...

Is that really a usable attack vector?

Isn't it actually more likely to leak kernel addresses to userspace
in the return stack buffer - which might be usable to get around KASR.

	David