linux-kernel - Re: [RFC PATCH] arch/x86: Optionally flush L1D on context switch

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Date:   Sun, 22 Mar 2020 05:10:43 +0000
From:   "Herrenschmidt, Benjamin" <benh@...zon.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Singh, Balbir" <sblbir@...zon.com>
CC:     "keescook@...omium.org" <keescook@...omium.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH] arch/x86: Optionally flush L1D on context switch

On Sat, 2020-03-21 at 11:05 +0100, Thomas Gleixner wrote:
> victim1
>  store secrit
>                         victim2
> attacker                  read secrit
> 
> Now if L1D is flushed on CPU0 before attacker reaches user space,
> i.e. reaches the attack code, then there is nothing to see. From the
> link:
> 
>   Similar to the L1TF VMM mitigations, snoop-assisted L1D sampling can be
>   mitigated by flushing the L1D cache between when secrets are accessed
>   and when possibly malicious software runs on the same core.
> 
> So the important point is to flush _before_ the attack code runs which
> involves going back to user space or guest mode.

So you mean switching from victim to attacker in the kernel, and going
back to victim before the attacker has a chance to actually execute any
userspace code ?

I can see why this doesn't require a flush, but is it a case worth
optimizing for either ?

IE. The flush in switch_mm is rather trivial, is lower overhead than
adding code to the userspace return code, and avoids kernel threads
likely, I prefer it for its simplicity TBH...

Cheers,
Ben.