KVM: RISC-V: reset smstateen CSRs

Not resetting smstateen is a potential security hole, because VU might
be able to access state that VS does not properly context-switch.

Fixes: 81f0f314fe ("RISCV: KVM: Add sstateen0 context save/restore")
Signed-off-by: Radim Krčmář <rkrcmar@ventanamicro.com>
Link: https://lore.kernel.org/r/20250403112522.1566629-8-rkrcmar@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/kvm/vcpu.c

@@ -77,6 +77,8 @@ static void kvm_riscv_reset_vcpu(struct kvm_vcpu *vcpu)
 	memcpy(cntx, reset_cntx, sizeof(*cntx));
 	spin_unlock(&vcpu->arch.reset_cntx_lock);
 
+	memset(&vcpu->arch.smstateen_csr, 0, sizeof(vcpu->arch.smstateen_csr));
+
 	kvm_riscv_vcpu_fp_reset(vcpu);
 
 	kvm_riscv_vcpu_vector_reset(vcpu);