arm64: KVM: perform save/restore of PAR_EL1

Not saving PAR_EL1 is an unfortunate oversight. If the guest
performs an AT* operation and gets scheduled out before reading
the result of the translation from PAREL1, it could become
corrupted by another guest or the host.

Saving this register is made slightly more complicated as KVM also
uses it on the permission fault handling path, leading to an ugly
"stash and restore" sequence. Fortunately, this is already a slow
path so we don't really care. Also, Linux doesn't do any AT*
operation, so Linux guests are not impacted by this bug.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/hyp.S

@@ -214,6 +214,7 @@ __kvm_hyp_code_start:
 	mrs	x21,	tpidr_el1
 	mrs	x22, 	amair_el1
 	mrs	x23, 	cntkctl_el1
+	mrs	x24,	par_el1
 
 	stp	x4, x5, [x3]
 	stp	x6, x7, [x3, #16]
@@ -225,6 +226,7 @@ __kvm_hyp_code_start:
 	stp	x18, x19, [x3, #112]
 	stp	x20, x21, [x3, #128]
 	stp	x22, x23, [x3, #144]
+	str	x24, [x3, #160]
 .endm
 
 .macro restore_sysregs
@@ -243,6 +245,7 @@ __kvm_hyp_code_start:
 	ldp	x18, x19, [x3, #112]
 	ldp	x20, x21, [x3, #128]
 	ldp	x22, x23, [x3, #144]
+	ldr	x24, [x3, #160]
 
 	msr	vmpidr_el2,	x4
 	msr	csselr_el1,	x5
@@ -264,6 +267,7 @@ __kvm_hyp_code_start:
 	msr	tpidr_el1,	x21
 	msr	amair_el1,	x22
 	msr	cntkctl_el1,	x23
+	msr	par_el1,	x24
 .endm
 
 .macro skip_32bit_state tmp, target
@@ -753,6 +757,10 @@ el1_trap:
 	 */
 	tbnz	x1, #7, 1f	// S1PTW is set
 
+	/* Preserve PAR_EL1 */
+	mrs	x3, par_el1
+	push	x3, xzr
+
 	/*
 	 * Permission fault, HPFAR_EL2 is invalid.
 	 * Resolve the IPA the hard way using the guest VA.
@@ -766,6 +774,8 @@ el1_trap:
 
 	/* Read result */
 	mrs	x3, par_el1
+	pop	x0, xzr			// Restore PAR_EL1 from the stack
+	msr	par_el1, x0
 	tbnz	x3, #0, 3f		// Bail out if we failed the translation
 	ubfx	x3, x3, #12, #36	// Extract IPA
 	lsl	x3, x3, #4		// and present it like HPFAR