[PATCH 6.1 178/303] KVM: Introduce vcpu->wants_to_run

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Matlack <[email protected]>

[ Upstream commit a6816314af5749cd88944bfdceb270c627cdf348 ]

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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>
Stable-dep-of: e800decd9c0a ("KVM: x86: Only reset TSC Deadline Timer in apic_timer_expired on KVM_RUN")
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 arch/arm64/kvm/arm.c       |    2 +-
 arch/mips/kvm/mips.c       |    2 +-
 arch/powerpc/kvm/powerpc.c |    2 +-
 arch/riscv/kvm/vcpu.c      |    2 +-
 arch/s390/kvm/kvm-s390.c   |    2 +-
 arch/x86/kvm/x86.c         |    4 ++--
 include/linux/kvm_host.h   |    1 +
 virt/kvm/kvm_main.c        |    3 +++
 8 files changed, 11 insertions(+), 7 deletions(-)

--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -891,7 +891,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 
 	vcpu_load(vcpu);
 
-	if (run->immediate_exit) {
+	if (!vcpu->wants_to_run) {
 		ret = -EINTR;
 		goto out;
 	}
--- a/arch/mips/kvm/mips.c
+++ b/arch/mips/kvm/mips.c
@@ -446,7 +446,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 		vcpu->mmio_needed = 0;
 	}
 
-	if (vcpu->run->immediate_exit)
+	if (!vcpu->wants_to_run)
 		goto out;
 
 	lose_fpu(1);
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -1866,7 +1866,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 
 	kvm_sigset_activate(vcpu);
 
-	if (run->immediate_exit)
+	if (!vcpu->wants_to_run)
 		r = -EINTR;
 	else
 		r = kvmppc_vcpu_run(vcpu);
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -1005,7 +1005,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 		return ret;
 	}
 
-	if (run->immediate_exit) {
+	if (!vcpu->wants_to_run) {
 		kvm_vcpu_srcu_read_unlock(vcpu);
 		return -EINTR;
 	}
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4881,7 +4881,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 	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 ||
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11264,7 +11264,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 
 	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;
 		}
@@ -11337,7 +11337,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_v
 		WARN_ON_ONCE(vcpu->mmio_needed);
 	}
 
-	if (kvm_run->immediate_exit) {
+	if (!vcpu->wants_to_run) {
 		r = -EINTR;
 		goto out;
 	}
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -365,6 +365,7 @@ struct kvm_vcpu {
 		bool dy_eligible;
 	} spin_loop;
 #endif
+	bool wants_to_run;
 	bool preempted;
 	bool ready;
 	struct kvm_vcpu_arch arch;
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4119,7 +4119,10 @@ static long kvm_vcpu_ioctl(struct file *
 				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;
 	}