KVM: arm64: Guard against NULL vcpu on VHE hyp panic path

On VHE, __hyp_call_panic() unconditionally calls __deactivate_traps(vcpu)
on the vcpu pointer read from host_ctxt->__hyp_running_vcpu. That pointer
is cleared after every guest exit (and is never set when no guest is
running), so an unexpected EL2 exception landing in _guest_exit_panic,
e.g. via the el2t*_invalid / el2h_irq_invalid vectors - reaches this
function with vcpu == NULL. __deactivate_traps() then dereferences vcpu
via ___deactivate_traps() -> vserror_state_is_nested() -> vcpu_has_nv()
-> vcpu->arch.features, faulting inside the panic handler and obscuring
the original failure.

The nVHE counterpart (hyp_panic() in arch/arm64/kvm/hyp/nvhe/switch.c)
already guards its vcpu-using cleanup with "if (vcpu)"; mirror that
here. sysreg_restore_host_state_vhe() does not depend on vcpu and
continues to run unconditionally, preserving panic forensics. The
trailing panic("...VCPU:%p", vcpu) prints "(null)" safely via printk's
%p handling.

Fixes: 6a0259e ("KVM: arm64: Remove hyp_panic arguments")
Assisted-by: Gemini:gemini-3.1-pro review-prompts
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260501112149.2824881-3-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

arch/arm64/kvm/hyp/vhe/switch.c

@@ -663,7 +663,8 @@ static void __noreturn __hyp_call_panic(u64 spsr, u64 elr, u64 par)
 	host_ctxt = host_data_ptr(host_ctxt);
 	vcpu = host_ctxt->__hyp_running_vcpu;
 
-	__deactivate_traps(vcpu);
+	if (vcpu)
+		__deactivate_traps(vcpu);
 	sysreg_restore_host_state_vhe(host_ctxt);
 
 	panic("HYP panic:\nPS:%08llx PC:%016llx ESR:%08llx\nFAR:%016llx HPFAR:%016llx PAR:%016llx\nVCPU:%p\n",