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

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Date:   Tue, 09 Jan 2018 00:55:42 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "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>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "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

On Mon, 2018-01-08 at 16:48 -0800, Linus Torvalds wrote:
> On Mon, Jan 8, 2018 at 4:42 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> > 
> > 
> > Hm... on a context switch you're reloading the registers that were in
> > the other saved context.
>
> Actually, iirc we used to very actively try to minimize that by having
> the inline asm mark a lot of registers as clobbered.

Sure, in that case it makes sense purely as a matter of hygiene to
explicitly clear the registers which were marked as clobbered.

In the original patch set from that Intel were collecting — before they
threw it all out the fucking window and started again from scratch on
the day the embargo broke — there were patches to do the same thing on
syscall entry too, for precisely the same reason.
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