Re: [PATCH v2 01/10] KVM: arm64: Block unsafe FF-A calls from the host

[Oliver Upton <oliver.upton@linux.dev>]

Hi Will,

On Wed, Apr 19, 2023 at 01:20:42PM +0100, Will Deacon wrote:

[...]

> +/*
> + * Is a given FFA function supported, either by forwarding on directly
> + * or by handling at EL2?
> + */
> +static bool ffa_call_supported(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:
> +	/* Don't advertise any features just yet */
> +	case FFA_FEATURES:
> +		return false;
> +	}
> +
> +	return true;
> +}

Apologies for rehashing something we dicussed in v1...

Enforcing the pKVM policy as a denylist rather than an allowlist
deserves a bit more elaboration, at least in the form of a comment. I
understand that we must trust EL3 by construction, but it is fuzzy why
it gets extended to what EL1 might do with FF-A calls that are unknown
to pKVM.

Broadening the scope for a moment, is my understanding correct that
limiting 'unknown' SMCs from host EL1 are an explicit non-goal of pKVM's
security model? Assuming a well-intentioned EL3, I'm just a bit worried
about any vendor-specific junkware that could be used by a malicious
EL1.

-- 
Thanks,
Oliver