Subject: Re: [PATCH 1/1] powerpc/kvm/book3s: Fixes possible 'use after release' of kvm

From: Paul Mackerras <[email-masked]>
Date: Thu, 28 Nov 2019 09:57:47 +1100

On Tue, Nov 26, 2019 at 02:52:12PM -0300, Leonardo Bras wrote:
> Fixes a possible 'use after free' of kvm variable.
> It does use mutex_unlock(&kvm->lock) after possible freeing a variable
> with kvm_put_kvm(kvm).

Comments below...

> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 5834db0a54c6..a402ead833b6 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -316,14 +316,13 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  
>  	if (ret >= 0)
>  		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
> -	else
> -		kvm_put_kvm(kvm);
>  
>  	mutex_unlock(&kvm->lock);
>  
>  	if (ret >= 0)
>  		return ret;
>  
> +	kvm_put_kvm(kvm);

There isn't a potential use-after-free here.  We are relying on the
property that the release function (kvm_vm_release) cannot be called
in parallel with this function.  The reason is that this function
(kvm_vm_ioctl_create_spapr_tce) is handling an ioctl on a kvm VM file
descriptor.  That means that a userspace process has the file
descriptor still open.  The code that implements the close() system
call makes sure that no thread is still executing inside any system
call that is using the same file descriptor before calling the file
descriptor's release function (in this case, kvm_vm_release).  That
means that this kvm_put_kvm() call here cannot make the reference
count go to zero.

>  	kfree(stt);
>   fail_acct:
>  	account_locked_vm(current->mm, kvmppc_stt_pages(npages), false);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 13efc291b1c7..f37089b60d09 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2744,10 +2744,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>  	/* Now it's all set up, let userspace reach it */
>  	kvm_get_kvm(kvm);
>  	r = create_vcpu_fd(vcpu);
> -	if (r < 0) {
> -		kvm_put_kvm(kvm);
> +	if (r < 0)
>  		goto unlock_vcpu_destroy;
> -	}
>  
>  	kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
>  
> @@ -2771,6 +2769,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>  	mutex_lock(&kvm->lock);
>  	kvm->created_vcpus--;
>  	mutex_unlock(&kvm->lock);
> +	if (r < 0)
> +		kvm_put_kvm(kvm);
>  	return r;
>  }

Once again we are inside an ioctl on the kvm VM file descriptor, so
the reference count cannot go to zero.

> @@ -3183,10 +3183,10 @@ static int kvm_ioctl_create_device(struct kvm *kvm,
>  	kvm_get_kvm(kvm);
>  	ret = anon_inode_getfd(ops->name, &kvm_device_fops, dev, O_RDWR | O_CLOEXEC);
>  	if (ret < 0) {
> -		kvm_put_kvm(kvm);
>  		mutex_lock(&kvm->lock);
>  		list_del(&dev->vm_node);
>  		mutex_unlock(&kvm->lock);
> +		kvm_put_kvm(kvm);
>  		ops->destroy(dev);
>  		return ret;
>  	}

Same again here.

Paul.