[PATCH 1/2] KVM: arm64: Prevent userspace from disabling AArch64 support at any virtualisable EL

[Marc Zyngier]

A sorry excuse for a selftest is trying to disable AArch64 support.
And yes, this goes as well as you can imagine.

Let's forbid this sort of things. Normal userspace shouldn't get
caught doing that.

Signed-off-by: Marc Zyngier <maz at kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 157de0ace6e7e..28dc778d0d9bb 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1946,6 +1946,12 @@ static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
 	if ((hw_val & mpam_mask) == (user_val & mpam_mask))
 		user_val &= ~ID_AA64PFR0_EL1_MPAM_MASK;
 
+	/* Fail the guest's request to disable the AA64 ISA at EL{0,1,2} */
+	if (!FIELD_GET(ID_AA64PFR0_EL1_EL0, user_val) ||
+	    !FIELD_GET(ID_AA64PFR0_EL1_EL1, user_val) ||
+	    (vcpu_has_nv(vcpu) && !FIELD_GET(ID_AA64PFR0_EL1_EL2, user_val)))
+		return -EINVAL;
+
 	return set_id_reg(vcpu, rd, user_val);
 }
 
-- 
2.39.2