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Date: Thu, 28 Mar 2024 13:59:00 +0000
From: Sebastian Ene <sebastianene@google.com>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: catalin.marinas@arm.com, james.morse@arm.com, jean-philippe@linaro.org,
	maz@kernel.org, qperret@google.com, qwandor@google.com,
	suzuki.poulose@arm.com, tabba@google.com, will@kernel.org,
	yuzenghui@huawei.com, kvmarm@lists.linux.dev,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	kernel-team@android.com
Subject: Re: [PATCH 2/2] KVM: arm64: Allow only the specified FF-A calls to
 be forwarded to TZ
Message-ID: <ZgV3pGvUhaK5otUL@google.com>
References: <20240322124303.309423-1-sebastianene@google.com>
 <20240322124303.309423-2-sebastianene@google.com>
 <Zf45eDs8Bd1UQ94Z@linux.dev>
 <ZgFgI0Ky2m4MUbw-@google.com>
 <ZgKKcurtndJaIwAV@linux.dev>
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On Tue, Mar 26, 2024 at 01:42:26AM -0700, Oliver Upton wrote:
> On Mon, Mar 25, 2024 at 11:29:39AM +0000, Sebastian Ene wrote:
> > On Fri, Mar 22, 2024 at 07:07:52PM -0700, Oliver Upton wrote:
> > > On Fri, Mar 22, 2024 at 12:43:03PM +0000, Sebastian Ene wrote:
> > > > The previous logic used a deny list to filter the FF-A calls. Because of
> > > > this, some of the calls escaped the check and they were forwarded by
> > > > default to Trustzone. (eg. FFA_MSG_SEND_DIRECT_REQ was denied but the 64
> > > > bit version of the call was not).
> > > > Modify the logic to use an allowlist and allow only the calls specified in
> > > > the filter function to be proxied to TZ from the hypervisor.
> > 
> > Hi Oliver,
> > 
> > > 
> > > I had discussed this with Will back when the feature was upstreamed and
> > > he said there's a lot of off-label calls that necessitate a denylist
> > > implementation. Has anything changed to give us confidence that we can
> > > be restrictive, at least on the FF-A range?
> > > 
> > 
> > I remember your proposal for having an allowlist instead. The current change makes
> > sense if we have https://lore.kernel.org/kvmarm/20240322124303.309423-1-sebastianene@google.com/
> > which opens the window for more FF-A calls to be forwarded to TZ.
> 
> Got it. Last time I didn't catch the level of abuse we expect to endure
> from vendors, but now it seems we will not support non-conforming calls
> that appear in standardized SMC ranges.
> 
> Adding mention of this to the changelog might be a good idea then.
> 

That's a good point. I didn't create a changelog for this but I should
add one and specify this.

> -- 
> Thanks,
> Oliver

Thanks,
Sebastian