KVM: Add helpers to wrap vcpu->srcu_idx and yell if it's abused

Add wrappers to acquire/release KVM's SRCU lock when stashing the index
in vcpu->src_idx, along with rudimentary detection of illegal usage,
e.g. re-acquiring SRCU and thus overwriting vcpu->src_idx.  Because the
SRCU index is (currently) either 0 or 1, illegal nesting bugs can go
unnoticed for quite some time and only cause problems when the nested
lock happens to get a different index.

Wrap the WARNs in PROVE_RCU=y, and make them ONCE, otherwise KVM will
likely yell so loudly that it will bring the kernel to its knees.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20220415004343.2203171-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 8 insertions, 8 deletions

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 9b30beac904d..af96dc0549a4 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -1334,11 +1334,11 @@ int kvm_s390_handle_wait(struct kvm_vcpu *vcpu)
 	hrtimer_start(&vcpu->arch.ckc_timer, sltime, HRTIMER_MODE_REL);
 	VCPU_EVENT(vcpu, 4, "enabled wait: %llu ns", sltime);
 no_timer:
-	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+	kvm_vcpu_srcu_read_unlock(vcpu);
 	kvm_vcpu_halt(vcpu);
 	vcpu->valid_wakeup = false;
 	__unset_cpu_idle(vcpu);
-	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+	kvm_vcpu_srcu_read_lock(vcpu);
 
 	hrtimer_cancel(&vcpu->arch.ckc_timer);
 	return 0;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 156d1c25a3c1..da3dabda1a12 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4237,14 +4237,14 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
 	 * We try to hold kvm->srcu during most of vcpu_run (except when run-
 	 * ning the guest), so that memslots (and other stuff) are protected
 	 */
-	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+	kvm_vcpu_srcu_read_lock(vcpu);
 
 	do {
 		rc = vcpu_pre_run(vcpu);
 		if (rc)
 			break;
 
-		srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+		kvm_vcpu_srcu_read_unlock(vcpu);
 		/*
 		 * As PF_VCPU will be used in fault handler, between
 		 * guest_enter and guest_exit should be no uaccess.
@@ -4281,12 +4281,12 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
 		__enable_cpu_timer_accounting(vcpu);
 		guest_exit_irqoff();
 		local_irq_enable();
-		vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+		kvm_vcpu_srcu_read_lock(vcpu);
 
 		rc = vcpu_post_run(vcpu, exit_reason);
 	} while (!signal_pending(current) && !guestdbg_exit_pending(vcpu) && !rc);
 
-	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+	kvm_vcpu_srcu_read_unlock(vcpu);
 	return rc;
 }
 
diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
index acda4b6fc851..dada78b92691 100644
--- a/arch/s390/kvm/vsie.c
+++ b/arch/s390/kvm/vsie.c
@@ -1091,7 +1091,7 @@ static int do_vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 
 	handle_last_fault(vcpu, vsie_page);
 
-	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+	kvm_vcpu_srcu_read_unlock(vcpu);
 
 	/* save current guest state of bp isolation override */
 	guest_bp_isolation = test_thread_flag(TIF_ISOLATE_BP_GUEST);
@@ -1133,7 +1133,7 @@ static int do_vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 	if (!guest_bp_isolation)
 		clear_thread_flag(TIF_ISOLATE_BP_GUEST);
 
-	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+	kvm_vcpu_srcu_read_lock(vcpu);
 
 	if (rc == -EINTR) {
 		VCPU_EVENT(vcpu, 3, "%s", "machine check");