During vCPU creation, acquire vcpu->mutex prior to exposing the vCPU to userspace, and hold the mutex until online_vcpus is bumped, i.e. until the vCPU is fully online from KVM's perspective. To ensure asynchronous vCPU ioctls also wait for the vCPU to come online, explicitly check online_vcpus at the start of kvm_vcpu_ioctl(), and take the vCPU's mutex to wait if necessary (having to wait for any ioctl should be exceedingly rare, i.e. not worth optimizing). Reported-by: Will Deacon <will@xxxxxxxxxx> Reported-by: Michal Luczaj <mhal@xxxxxxx> Link: https://lore.kernel.org/all/20240730155646.1687-1-will@xxxxxxxxxx Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- virt/kvm/kvm_main.c | 47 ++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 46 insertions(+), 1 deletion(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 05cbb2548d99..fca9f74e9544 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -4287,7 +4287,14 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) if (r) goto unlock_vcpu_destroy; - /* Now it's all set up, let userspace reach it */ + /* + * Now it's all set up, let userspace reach it. Grab the vCPU's mutex + * so that userspace can't invoke vCPU ioctl()s until the vCPU is fully + * visible (per online_vcpus), e.g. so that KVM doesn't get tricked + * into a NULL-pointer dereference because KVM thinks the _current_ + * vCPU doesn't exist. + */ + mutex_lock(&vcpu->mutex); kvm_get_kvm(kvm); r = create_vcpu_fd(vcpu); if (r < 0) @@ -4304,6 +4311,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) */ smp_wmb(); atomic_inc(&kvm->online_vcpus); + mutex_unlock(&vcpu->mutex); mutex_unlock(&kvm->lock); kvm_arch_vcpu_postcreate(vcpu); @@ -4311,6 +4319,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id) return r; kvm_put_xa_release: + mutex_unlock(&vcpu->mutex); kvm_put_kvm_no_destroy(kvm); xa_release(&kvm->vcpu_array, vcpu->vcpu_idx); unlock_vcpu_destroy: @@ -4437,6 +4446,33 @@ static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu, } #endif +static int kvm_wait_for_vcpu_online(struct kvm_vcpu *vcpu) +{ + struct kvm *kvm = vcpu->kvm; + + /* + * In practice, this happy path will always be taken, as a well-behaved + * VMM will never invoke a vCPU ioctl() before KVM_CREATE_VCPU returns. + */ + if (likely(vcpu->vcpu_idx < atomic_read(&kvm->online_vcpus))) + return 0; + + /* + * Acquire and release the vCPU's mutex to wait for vCPU creation to + * complete (kvm_vm_ioctl_create_vcpu() holds the mutex until the vCPU + * is fully online). + */ + if (mutex_lock_killable(&vcpu->mutex)) + return -EINTR; + + mutex_unlock(&vcpu->mutex); + + if (WARN_ON_ONCE(!kvm_get_vcpu(kvm, vcpu->vcpu_idx))) + return -EIO; + + return 0; +} + static long kvm_vcpu_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg) { @@ -4452,6 +4488,15 @@ static long kvm_vcpu_ioctl(struct file *filp, if (unlikely(_IOC_TYPE(ioctl) != KVMIO)) return -EINVAL; + /* + * Wait for the vCPU to be online before handling the ioctl(), as KVM + * assumes the vCPU is reachable via vcpu_array, i.e. may dereference + * a NULL pointer if userspace invokes an ioctl() before KVM is ready. + */ + r = kvm_wait_for_vcpu_online(vcpu); + if (r) + return r; + /* * Some architectures have vcpu ioctls that are asynchronous to vcpu * execution; mutex_lock() would break them. -- 2.47.0.rc0.187.ge670bccf7e-goog
