linux-kernel - Re: [PATCH 03/12] KVM: arm64: Block unsafe FF-A calls from the host

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Date:   Wed, 16 Nov 2022 17:48:42 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Andrew Walbran <qwandor@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 03/12] KVM: arm64: Block unsafe FF-A calls from the host

Sorry, hit send a bit too early. Reviewing the patch itself:

On Wed, Nov 16, 2022 at 05:03:26PM +0000, Quentin Perret wrote:

[...]

> +static bool ffa_call_unsupported(u64 func_id)
> +{
> +	switch (func_id) {
> +	/* Unsupported memory management calls */
> +	case FFA_FN64_MEM_RETRIEVE_REQ:
> +	case FFA_MEM_RETRIEVE_RESP:
> +	case FFA_MEM_RELINQUISH:
> +	case FFA_MEM_OP_PAUSE:
> +	case FFA_MEM_OP_RESUME:
> +	case FFA_MEM_FRAG_RX:
> +	case FFA_FN64_MEM_DONATE:
> +	/* Indirect message passing via RX/TX buffers */
> +	case FFA_MSG_SEND:
> +	case FFA_MSG_POLL:
> +	case FFA_MSG_WAIT:
> +	/* 32-bit variants of 64-bit calls */
> +	case FFA_MSG_SEND_DIRECT_REQ:
> +	case FFA_MSG_SEND_DIRECT_RESP:
> +	case FFA_RXTX_MAP:
> +	case FFA_MEM_DONATE:
> +	case FFA_MEM_RETRIEVE_REQ:
> +		return true;
> +	}
> +
> +	return false;
> +}

Wouldn't an allowlist behave better in this case? While unlikely, you
wouldn't want EL3 implementing some FFA_BACKDOOR_PVM SMC that falls
outside of the denylist and is passed through.

> +bool kvm_host_ffa_handler(struct kvm_cpu_context *host_ctxt)
> +{
> +	DECLARE_REG(u64, func_id, host_ctxt, 0);
> +	struct arm_smccc_res res;
> +
> +	if (!is_ffa_call(func_id))
> +		return false;
> +
> +	switch (func_id) {
> +	/* Memory management */
> +	case FFA_FN64_RXTX_MAP:
> +	case FFA_RXTX_UNMAP:
> +	case FFA_MEM_SHARE:
> +	case FFA_FN64_MEM_SHARE:
> +	case FFA_MEM_LEND:
> +	case FFA_FN64_MEM_LEND:
> +	case FFA_MEM_RECLAIM:
> +	case FFA_MEM_FRAG_TX:
> +		break;
> +	}

What is the purpose of this switch?

> +
> +	if (!ffa_call_unsupported(func_id))
> +		return false; /* Pass through */

Another (tiny) benefit of implementing an allowlist is that it avoids
the use of double-negative logic like this.

--
Thanks,
Oliver