KVM: Introduce vcpu->wants_to_run

Introduce vcpu->wants_to_run to indicate when a vCPU is in its core run
loop, i.e. when the vCPU is running the KVM_RUN ioctl and immediate_exit
was not set.

Replace all references to vcpu->run->immediate_exit with
!vcpu->wants_to_run to avoid TOCTOU races with userspace. For example, a
malicious userspace could invoked KVM_RUN with immediate_exit=true and
then after KVM reads it to set wants_to_run=false, flip it to false.
This would result in the vCPU running in KVM_RUN with
wants_to_run=false. This wouldn't cause any real bugs today but is a
dangerous landmine.

Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240503181734.1467938-2-dmatlack@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

9 files changed, 12 insertions, 8 deletions

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9996a989b52e..f9a418e9ea6a 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1099,7 +1099,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 
 	vcpu_load(vcpu);
 
-	if (run->immediate_exit) {
+	if (!vcpu->wants_to_run) {
 		ret = -EINTR;
 		goto out;
 	}
diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
index 9e8030d45129..f27d2bf14dc0 100644
--- a/arch/loongarch/kvm/vcpu.c
+++ b/arch/loongarch/kvm/vcpu.c
@@ -1266,7 +1266,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 			kvm_complete_iocsr_read(vcpu, run);
 	}
 
-	if (run->immediate_exit)
+	if (!vcpu->wants_to_run)
 		return r;
 
 	/* Clear exit_reason */
diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
index 231ac052b506..f1a99962027a 100644
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -436,7 +436,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		vcpu->mmio_needed = 0;
 	}
 
-	if (vcpu->run->immediate_exit)
+	if (!vcpu->wants_to_run)
 		goto out;
 
 	lose_fpu(1);
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index d32abe7fe6ab..961aadc71de2 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -1852,7 +1852,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 
 	kvm_sigset_activate(vcpu);
 
-	if (run->immediate_exit)
+	if (!vcpu->wants_to_run)
 		r = -EINTR;
 	else
 		r = kvmppc_vcpu_run(vcpu);
diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index 17e21df36cc1..2123df6ee409 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -760,7 +760,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		return ret;
 	}
 
-	if (run->immediate_exit) {
+	if (!vcpu->wants_to_run) {
 		kvm_vcpu_srcu_read_unlock(vcpu);
 		return -EINTR;
 	}
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 4641083ee100..5da808976085 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -5026,7 +5026,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 	if (vcpu->kvm->arch.pv.dumping)
 		return -EINVAL;
 
-	if (kvm_run->immediate_exit)
+	if (!vcpu->wants_to_run)
 		return -EINTR;
 
 	if (kvm_run->kvm_valid_regs & ~KVM_SYNC_S390_VALID_FIELDS ||
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 994aa281b07d..de581487e3c9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11407,7 +11407,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 
 	kvm_vcpu_srcu_read_lock(vcpu);
 	if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
-		if (kvm_run->immediate_exit) {
+		if (!vcpu->wants_to_run) {
 			r = -EINTR;
 			goto out;
 		}
@@ -11485,7 +11485,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		WARN_ON_ONCE(vcpu->mmio_needed);
 	}
 
-	if (kvm_run->immediate_exit) {
+	if (!vcpu->wants_to_run) {
 		r = -EINTR;
 		goto out;
 	}
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 7b9d2633a931..d72ced3e74d1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -378,6 +378,7 @@ struct kvm_vcpu {
 		bool dy_eligible;
 	} spin_loop;
 #endif
+	bool wants_to_run;
 	bool preempted;
 	bool ready;
 	bool scheduled_out;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index fed80bfbe99b..66bfdfa11805 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4435,7 +4435,10 @@ static long kvm_vcpu_ioctl(struct file *filp,
 				synchronize_rcu();
 			put_pid(oldpid);
 		}
+		vcpu->wants_to_run = !READ_ONCE(vcpu->run->immediate_exit);
 		r = kvm_arch_vcpu_ioctl_run(vcpu);
+		vcpu->wants_to_run = false;
+
 		trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
 		break;
 	}