KVM: arm64: Restore hyp when panicking in guest context

If the guest context is loaded when a panic is triggered, restore the
hyp context so e.g. the shadow call stack works when hyp_panic() is
called and SP_EL0 is valid when the host's panic() is called.

Use the hyp context's __hyp_running_vcpu field to track when hyp
transitions to and from the guest vcpu so the exception handlers know
whether the context needs to be restored.

Signed-off-by: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200915104643.2543892-11-ascull@google.com

arch/arm64/include/asm/kvm_asm.h

@@ -236,6 +236,16 @@ extern char __smccc_workaround_1_smc[__SMCCC_WORKAROUND_1_SMC_SZ];
 	ldr	\vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
 .endm
 
+.macro get_loaded_vcpu vcpu, ctxt
+	hyp_adr_this_cpu \ctxt, kvm_hyp_ctxt, \vcpu
+	ldr	\vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
+.endm
+
+.macro set_loaded_vcpu vcpu, ctxt, tmp
+	hyp_adr_this_cpu \ctxt, kvm_hyp_ctxt, \tmp
+	str	\vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
+.endm
+
 /*
  * KVM extable for unexpected exceptions.
  * In the same format _asm_extable, but output to a different section so that

arch/arm64/kvm/hyp/entry.S

@@ -86,6 +86,8 @@ alternative_else_nop_endif
 	ret
 
 1:
+	set_loaded_vcpu x0, x1, x2
+
 	add	x29, x0, #VCPU_CONTEXT
 
 	// Macro ptrauth_switch_to_guest format:
@@ -116,6 +118,26 @@ alternative_else_nop_endif
 	eret
 	sb
 
+SYM_INNER_LABEL(__guest_exit_panic, SYM_L_GLOBAL)
+	// x2-x29,lr: vcpu regs
+	// vcpu x0-x1 on the stack
+
+	// If the hyp context is loaded, go straight to hyp_panic
+	get_loaded_vcpu x0, x1
+	cbz	x0, hyp_panic
+
+	// The hyp context is saved so make sure it is restored to allow
+	// hyp_panic to run at hyp and, subsequently, panic to run in the host.
+	// This makes use of __guest_exit to avoid duplication but sets the
+	// return address to tail call into hyp_panic. As a side effect, the
+	// current state is saved to the guest context but it will only be
+	// accurate if the guest had been completely restored.
+	hyp_adr_this_cpu x0, kvm_hyp_ctxt, x1
+	adr	x1, hyp_panic
+	str	x1, [x0, #CPU_XREG_OFFSET(30)]
+
+	get_vcpu_ptr	x1, x0
+
 SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
 	// x0: return code
 	// x1: vcpu
@@ -163,6 +185,8 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
 	// Now restore the hyp regs
 	restore_callee_saved_regs x2
 
+	set_loaded_vcpu xzr, x1, x2
+
 alternative_if ARM64_HAS_RAS_EXTN
 	// If we have the RAS extensions we can consume a pending error
 	// without an unmask-SError and isb. The ESB-instruction consumed any

arch/arm64/kvm/hyp/hyp-entry.S

@@ -145,7 +145,7 @@ el2_error:
 	eret
 	sb
 
-.macro invalid_vector	label, target = hyp_panic
+.macro invalid_vector	label, target = __guest_exit_panic
 	.align	2
 SYM_CODE_START(\label)
 	b \target
@@ -186,10 +186,9 @@ check_preamble_length 661b, 662b
 .macro invalid_vect target
 	.align 7
 661:
-	b	\target
 	nop
+	stp	x0, x1, [sp, #-16]!
 662:
-	ldp	x0, x1, [sp], #16
 	b	\target
 
 check_preamble_length 661b, 662b

arch/arm64/kvm/hyp/include/hyp/switch.h

@@ -509,6 +509,7 @@ static inline void __set_host_arch_workaround_state(struct kvm_vcpu *vcpu)
 
 static inline void __kvm_unexpected_el2_exception(void)
 {
+	extern char __guest_exit_panic[];
 	unsigned long addr, fixup;
 	struct exception_table_entry *entry, *end;
 	unsigned long elr_el2 = read_sysreg(elr_el2);
@@ -529,7 +530,8 @@ static inline void __kvm_unexpected_el2_exception(void)
 		return;
 	}
 
-	hyp_panic();
+	/* Trigger a panic after restoring the hyp context. */
+	write_sysreg(__guest_exit_panic, elr_el2);
 }
 
 #endif /* __ARM64_KVM_HYP_SWITCH_H__ */

arch/arm64/kvm/hyp/nvhe/host.S

@@ -75,6 +75,11 @@ SYM_FUNC_END(__hyp_do_panic)
 
 .macro invalid_host_vect
 	.align 7
+	/* If a guest is loaded, panic out of it. */
+	stp	x0, x1, [sp, #-16]!
+	get_loaded_vcpu x0, x1
+	cbnz	x0, __guest_exit_panic
+	add	sp, sp, #16
 	b	hyp_panic
 .endm