linux-kernel - Re: [PATCH 5/7] x86: Use IBRS for firmware update path

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Message-ID: <20180104200515.GC30228@kroah.com>
Date:   Thu, 4 Jan 2018 21:05:15 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        linux-kernel@...r.kernel.org, David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH 5/7] x86: Use IBRS for firmware update path

On Thu, Jan 04, 2018 at 09:56:46AM -0800, Tim Chen wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> We are impervious to the indirect branch prediction attack with retpoline
> but firmware won't be, so we still need to set IBRS to protect
> firmware code execution when calling into firmware at runtime.

Wait, what?

Maybe it's just the wine from dinner talking, but if the firmware has
issues, we have bigger things to worry about here, right?  It already
handed over the "chain of trust" to us, so we have already implicitly
trusted that the firmware was correct here.  So why do we need to do
anything about firmware calls in this manner?

Or am I totally missing something else here?

thanks,

greg k-h