KVM: arm64: Use shadow SPSR_EL1 when injecting exceptions on !VHE

Injecting an exception into a guest with non-VHE is risky business.
Instead of writing in the shadow register for the switch code to
restore it, we override the CPU register instead. Which gets
overriden a few instructions later by said restore code.

The result is that although the guest correctly gets the exception,
it will return to the original context in some random state,
depending on what was there the first place... Boo.

Fix the issue by writing to the shadow register. The original code
is absolutely fine on VHE, as the state is already loaded, and writing
to the shadow register in that case would actually be a bug.

Fixes: bb666c47 ("KVM: arm64: Inject AArch64 exceptions from HYP")
Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/r/20220121184207.423426-1-maz@kernel.org

arch/arm64/kvm/hyp/exception.c

@@ -38,7 +38,10 @@ static inline void __vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64 val, int reg)
 
 static void __vcpu_write_spsr(struct kvm_vcpu *vcpu, u64 val)
 {
-	write_sysreg_el1(val, SYS_SPSR);
+	if (has_vhe())
+		write_sysreg_el1(val, SYS_SPSR);
+	else
+		__vcpu_sys_reg(vcpu, SPSR_EL1) = val;
 }
 
 static void __vcpu_write_spsr_abt(struct kvm_vcpu *vcpu, u64 val)