linux-kernel - Re: [PATCH v2 3/4] arch/x86: Optionally flush L1D on context switch

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Message-ID: <728ba30fdc269d4b24c4fb16832e0151e8270cba.camel@amazon.com>
Date:   Tue, 7 Apr 2020 23:39:14 +0000
From:   "Singh, Balbir" <sblbir@...zon.com>
To:     "keescook@...omium.org" <keescook@...omium.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>
Subject: Re: [PATCH v2 3/4] arch/x86: Optionally flush L1D on context switch

On Tue, 2020-04-07 at 11:26 -0700, Kees Cook wrote:
> 
> 
> On Mon, Apr 06, 2020 at 01:19:45PM +1000, Balbir Singh wrote:
> > Implement a mechanism to selectively flush the L1D cache. The goal is to
> > allow tasks that are paranoid due to the recent snoop assisted data
> > sampling
> > vulnerabilites, to flush their L1D on being switched out.  This protects
> > their data from being snooped or leaked via side channels after the task
> > has context switched out.
> > 
> > There are two scenarios we might want to protect against, a task leaving
> > the CPU with data still in L1D (which is the main concern of this patch),
> > the second scenario is a malicious task coming in (not so well trusted)
> > for which we want to clean up the cache before it starts. Only the case
> > for the former is addressed.
> > 
> > Add arch specific prctl()'s to opt-in to the L1D cache on context switch
> > out, the existing mechanisms of tracking prev_mm via cpu_tlbstate is
> > reused. cond_ibpb() is refactored and renamed into cond_mitigation().
> 
> I still think this should be a generic prctl(). If there is a strong
> reason not to do this, can it be described in the commit log here?
> 
> -Kees
> 

I can move to prctl() if that is what you prefer, the prctl() can then do arch
specific things. I thought in my question around would other arch's like to do
this, I did not hear anything specific, but I am happy to convert the
interface over.

Balbir Singh.