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

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Message-ID: <f3750cf8-88fc-cae7-1cfb-cb4b86b44704@redhat.com>
Date:   Wed, 27 Nov 2019 17:40:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/1] powerpc/kvm/book3s: Fixes possible 'use after
 release' of kvm

On 26/11/19 18:52, 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).
> 
> Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_64_vio.c | 3 +--
>  virt/kvm/kvm_main.c              | 8 ++++----
>  2 files changed, 5 insertions(+), 6 deletions(-)
> 
> 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);
>  	kfree(stt);
>   fail_acct:
>  	account_locked_vm(current->mm, kvmppc_stt_pages(npages), false);

This part is a good change, as it makes the code clearer.  The
virt/kvm/kvm_main.c bits, however, are not necessary as explained by Sean.

Paolo

> 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;
>  }
>  
> @@ -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;
>  	}
>