KVM: arm/arm64: VGIC/ITS: protect kvm_read_guest() calls with SRCU lock

kvm_read_guest() will eventually look up in kvm_memslots(), which requires
either to hold the kvm->slots_lock or to be inside a kvm->srcu critical
section.
In contrast to x86 and s390 we don't take the SRCU lock on every guest
exit, so we have to do it individually for each kvm_read_guest() call.

Provide a wrapper which does that and use that everywhere.

Note that ending the SRCU critical section before returning from the
kvm_read_guest() wrapper is safe, because the data has been *copied*, so
we don't need to rely on valid references to the memslot anymore.

Cc: Stable <stable@vger.kernel.org> # 4.8+
Reported-by: Jan Glauber <jan.glauber@caviumnetworks.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/arm/include/asm/kvm_mmu.h

@@ -309,6 +309,22 @@ static inline unsigned int kvm_get_vmid_bits(void)
 	return 8;
 }
 
+/*
+ * We are not in the kvm->srcu critical section most of the time, so we take
+ * the SRCU read lock here. Since we copy the data from the user page, we
+ * can immediately drop the lock again.
+ */
+static inline int kvm_read_guest_lock(struct kvm *kvm,
+				      gpa_t gpa, void *data, unsigned long len)
+{
+	int srcu_idx = srcu_read_lock(&kvm->srcu);
+	int ret = kvm_read_guest(kvm, gpa, data, len);
+
+	srcu_read_unlock(&kvm->srcu, srcu_idx);
+
+	return ret;
+}
+
 static inline void *kvm_get_hyp_vector(void)
 {
 	return kvm_ksym_ref(__kvm_hyp_vector);

arch/arm64/include/asm/kvm_mmu.h

@@ -360,6 +360,22 @@ static inline unsigned int kvm_get_vmid_bits(void)
 	return (cpuid_feature_extract_unsigned_field(reg, ID_AA64MMFR1_VMIDBITS_SHIFT) == 2) ? 16 : 8;
 }
 
+/*
+ * We are not in the kvm->srcu critical section most of the time, so we take
+ * the SRCU read lock here. Since we copy the data from the user page, we
+ * can immediately drop the lock again.
+ */
+static inline int kvm_read_guest_lock(struct kvm *kvm,
+				      gpa_t gpa, void *data, unsigned long len)
+{
+	int srcu_idx = srcu_read_lock(&kvm->srcu);
+	int ret = kvm_read_guest(kvm, gpa, data, len);
+
+	srcu_read_unlock(&kvm->srcu, srcu_idx);
+
+	return ret;
+}
+
 #ifdef CONFIG_KVM_INDIRECT_VECTORS
 /*
  * EL2 vectors can be mapped and rerouted in a number of ways,

virt/kvm/arm/vgic/vgic-its.c

@@ -281,7 +281,7 @@ static int update_lpi_config(struct kvm *kvm, struct vgic_irq *irq,
 	int ret;
 	unsigned long flags;
 
-	ret = kvm_read_guest(kvm, propbase + irq->intid - GIC_LPI_OFFSET,
+	ret = kvm_read_guest_lock(kvm, propbase + irq->intid - GIC_LPI_OFFSET,
 				  &prop, 1);
 
 	if (ret)
@@ -444,7 +444,8 @@ static int its_sync_lpi_pending_table(struct kvm_vcpu *vcpu)
 		 * this very same byte in the last iteration. Reuse that.
 		 */
 		if (byte_offset != last_byte_offset) {
-			ret = kvm_read_guest(vcpu->kvm, pendbase + byte_offset,
+			ret = kvm_read_guest_lock(vcpu->kvm,
+						  pendbase + byte_offset,
 						  &pendmask, 1);
 			if (ret) {
 				kfree(intids);
@@ -789,7 +790,7 @@ static bool vgic_its_check_id(struct vgic_its *its, u64 baser, u32 id,
 		return false;
 
 	/* Each 1st level entry is represented by a 64-bit value. */
-	if (kvm_read_guest(its->dev->kvm,
+	if (kvm_read_guest_lock(its->dev->kvm,
 			   BASER_ADDRESS(baser) + index * sizeof(indirect_ptr),
 			   &indirect_ptr, sizeof(indirect_ptr)))
 		return false;
@@ -1370,7 +1371,7 @@ static void vgic_its_process_commands(struct kvm *kvm, struct vgic_its *its)
 	cbaser = CBASER_ADDRESS(its->cbaser);
 
 	while (its->cwriter != its->creadr) {
-		int ret = kvm_read_guest(kvm, cbaser + its->creadr,
+		int ret = kvm_read_guest_lock(kvm, cbaser + its->creadr,
 					      cmd_buf, ITS_CMD_SIZE);
 		/*
 		 * If kvm_read_guest() fails, this could be due to the guest