Re: [PATCH v4 5/6] Optionally flush L1D on context switch

From: "Singh, Balbir" <sblbir@amazon.com>
To: "keescook@chromium.org" <keescook@chromium.org>
Cc: "tglx@linutronix.de" <tglx@linutronix.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"tony.luck@intel.com" <tony.luck@intel.com>,
	"benh@kernel.crashing.org" <benh@kernel.crashing.org>,
	"jpoimboe@redhat.com" <jpoimboe@redhat.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"dave.hansen@intel.com" <dave.hansen@intel.com>
Subject: Re: [PATCH v4 5/6] Optionally flush L1D on context switch
Date: Fri, 24 Apr 2020 09:56:39 +0000	[thread overview]
Message-ID: <5ee183ddd5fd9413353190fe95058a8be74c7d55.camel@amazon.com> (raw)
In-Reply-To: <202004231219.9D614F5@keescook>

On Thu, 2020-04-23 at 12:19 -0700, Kees Cook wrote:
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> 
> 
> On Fri, Apr 24, 2020 at 12:01:24AM +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.
> > 
> > A new thread_info flag TIF_SPEC_FLUSH_L1D is added to track tasks which
> > opt-into L1D flushing. cpu_tlbstate.last_user_mm_spec is used to convert
> > the TIF flags into mm state (per cpu via last_user_mm_spec) in
> > cond_mitigation(), which then used to do decide when to call flush_l1d().
> > 
> > Add 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 to track state of the tasks and to flush the L1D cache.
> > The prctl interface is generic and can be ported over to other
> > architectures.
> > 
> > Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Balbir Singh <sblbir@amazon.com>
> 
> I'm not a huge fan of __weak (I like CONFIGs better), but that's no
> enough to NAK this. ;) Thanks for the prctl() change!
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> 

Thanks, the CONFIG_* seemed a bit much for two functions.

Balbir Singh.