linux-kernel - [PATCH 1/3] KVM: arm64: Restrict FF-A host version renegotiation

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Message-ID: <CA+AY4XcaJa1_U3qXQUBj4wZJYc9hKmRX8FTNeDvV2auEnC_WrA@mail.gmail.com>
Date: Thu, 1 May 2025 20:52:39 -0700
From: Per Larsen <perl@...unant.com>
To: linux-kernel@...r.kernel.org
Cc: "qperret@...gle.com" <qperret@...gle.com>, "sebastianene@...gle.com" <sebastianene@...gle.com>, kernel-team@...roid.com, 
	"will@...nel.org" <will@...nel.org>, maz@...nel.org, 
	"sudeep.holla@....com" <sudeep.holla@....com>, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, "catalin.marinas@....com" <catalin.marinas@....com>, yuzenghui@...wei.com, 
	Armelle Laine <armellel@...gle.com>, arve@...roid.org
Subject: [PATCH 1/3] KVM: arm64: Restrict FF-A host version renegotiation

FF-A implementations with the same major version must interoperate with
earlier minor versions per DEN0077A 1.2 REL0 13.2.1 but FF-A version 1.1
broke the ABI on several structures and 1.2 relies on SMCCC 1.2 is not
backwards compatible with SMCCC 1.2 (see DEN0028 1.6 G BET0 Appendix F).

If we return the negotiated hypervisor version when the host requests a
lesser minor version, the host will rely on the FF-A interoperability
rules. Since the hypervisor does not currently have the necessary
compatibility paths (e.g. to handle breaking changes to the SMC calling
convention), return NOT_SUPPORTED.

Signed-off-by: Per Larsen <perlarsen@...gle.com>
Signed-off-by: Per Larsen <perl@...unant.com>
---
 arch/arm64/kvm/hyp/nvhe/ffa.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
index 3369dd0c4009..10e88207b78e 100644
--- a/arch/arm64/kvm/hyp/nvhe/ffa.c
+++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
@@ -712,7 +712,24 @@ static void do_ffa_version(struct arm_smccc_res *res,

  hyp_spin_lock(&version_lock);
  if (has_version_negotiated) {
- res->a0 = hyp_ffa_version;
+ /*
+ * FF-A implementations with the same major version must
+ * interoperate with earlier minor versions per DEN0077A 1.2
+ * REL0 13.2.1 but FF-A version 1.1 broke the ABI on several
+ * structures and 1.2 relies on SMCCC 1.2 is not backwards
+ * compatible with SMCCC 1.2 (see DEN0028 1.6 G BET0 Appendix F).
+ *
+ * If we return the negotiated hypervisor version when the host
+ * requests a lesser minor version, the host will rely on the
+ * aforementioned FF-A interoperability rules. Since the
+ * hypervisor does not currently have the necessary compatibility
+ * paths (e.g. to paper over the above-mentioned calling
+ * convention changes), return NOT_SUPPORTED.
+ */
+ if (FFA_MINOR_VERSION(ffa_req_version) < FFA_MINOR_VERSION(hyp_ffa_version))
+ res->a0 = FFA_RET_NOT_SUPPORTED;
+ else
+ res->a0 = hyp_ffa_version;
  goto unlock;
  }

-- 
2.49.0.906.g1f30a19c02-goog