KVM: arm64: nVHE: Handle hyp panics

Restore the host context when panicking from hyp to give the best chance
of the panic being clean.

The host requires that registers be preserved such as x18 for the shadow
callstack. If the panic is caused by an exception from EL1, the host
context is still valid so the panic can return straight back to the
host. If the panic comes from EL2 then it's most likely that the hyp
context is active and the host context needs to be restored.

There are windows before and after the host context is saved and
restored that restoration is attempted incorrectly and the panic won't
be clean.

Signed-off-by: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200915104643.2543892-14-ascull@google.com

1 files changed, 5 insertions, 13 deletions

diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index c99945cda779..29febf9a93f2 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -242,6 +242,8 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
 	if (system_uses_irq_prio_masking())
 		gic_write_pmr(GIC_PRIO_IRQOFF);
 
+	host_ctxt->__hyp_running_vcpu = NULL;
+
 	return exit_code;
 }
 
@@ -250,31 +252,21 @@ void __noreturn hyp_panic(void)
 	u64 spsr = read_sysreg_el2(SYS_SPSR);
 	u64 elr = read_sysreg_el2(SYS_ELR);
 	u64 par = read_sysreg(par_el1);
+	bool restore_host = true;
 	struct kvm_cpu_context *host_ctxt;
 	struct kvm_vcpu *vcpu;
-	unsigned long str_va;
 
 	host_ctxt = &__hyp_this_cpu_ptr(kvm_host_data)->host_ctxt;
 	vcpu = host_ctxt->__hyp_running_vcpu;
 
-	if (read_sysreg(vttbr_el2)) {
+	if (vcpu) {
 		__timer_disable_traps(vcpu);
 		__deactivate_traps(vcpu);
 		__load_host_stage2();
 		__sysreg_restore_state_nvhe(host_ctxt);
 	}
 
-	/*
-	 * Force the panic string to be loaded from the literal pool,
-	 * making sure it is a kernel address and not a PC-relative
-	 * reference.
-	 */
-	asm volatile("ldr %0, =%1" : "=r" (str_va) : "S" (__hyp_panic_string));
-
-	__hyp_do_panic(str_va,
-		       spsr, elr,
-		       read_sysreg(esr_el2), read_sysreg_el2(SYS_FAR),
-		       read_sysreg(hpfar_el2), par, vcpu);
+	__hyp_do_panic(restore_host, spsr, elr, par);
 	unreachable();
 }