Re: [PATCH] KVM: release anon file in failure path of vm creation

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/07/2016 18:46, Al Viro wrote:
> On Tue, Jul 12, 2016 at 12:24:39PM +0200, Paolo Bonzini wrote:
> 
>> On 12/07/2016 11:38, Liu Shuo wrote:
>>> The failure of create debugfs of VM will return directly without release
>>> the anon file. It will leak memory and file descriptors, even through
>>> be not serious.
>>>
>>> Signed-off-by: Liu Shuo <shuo.a.liu@intel.com>
> 
> This is broken.
> 
>>> @@ -3067,6 +3068,7 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
>>>  
>>>  	if (kvm_create_vm_debugfs(kvm, r) < 0) {
>>>  		kvm_put_kvm(kvm);
>>> +		sys_close(r);
> 
> You have no warranty whatsoever that descriptor table has not been changed
> by that point.  You should *NEVER* use sys_close() on failure exit paths
> like that.  Moreover, this kvm_put_kvm() becomes a double-put, since
> closing the damn file will drop that reference to kvm.
> 
>>>  		return -ENOMEM;
>>>  	}
>>>  
>>>
>>
>> Thanks, applied to kvm/master.
> 
> Please, revert.  anon_inode_getfd() should be used only when there's no
> possible failures past its call.  What you need in this case (due to fucking
> ugly API - decriptor number used as part of debugfs pathname) is something
> like this (against mainline):

Al,

thanks very much for the explanation and the patch.  I've reverted the
original patch and installed yours instead.

Paolo

> [kvm] don't use anon_inode_getfd() before possible failures
> 
> Once anon_inode_getfd() has succeeded, it's impossible to undo in a clean
> way and no, sys_close() is not usable in such cases.  Use anon_inode_getfile()
> and get_unused_fd_flags() to get struct file and descriptor and do *not*
> install the file into descriptor table until after the last possible failure
> exit.
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 48bd520..7f82c6c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3048,6 +3048,7 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
>  {
>  	int r;
>  	struct kvm *kvm;
> +	struct file *file;
>  
>  	kvm = kvm_create_vm(type);
>  	if (IS_ERR(kvm))
> @@ -3059,17 +3060,25 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
>  		return r;
>  	}
>  #endif
> -	r = anon_inode_getfd("kvm-vm", &kvm_vm_fops, kvm, O_RDWR | O_CLOEXEC);
> +	r = get_unused_fd_flags(O_CLOEXEC);
>  	if (r < 0) {
>  		kvm_put_kvm(kvm);
>  		return r;
>  	}
> +	file = anon_inode_getfile("kvm-vm", &kvm_vm_fops, kvm, O_RDWR);
> +	if (IS_ERR(file)) {
> +		put_unused_fd(r);
> +		kvm_put_kvm(kvm);
> +		return PTR_ERR(file);
> +	}
>  
>  	if (kvm_create_vm_debugfs(kvm, r) < 0) {
> -		kvm_put_kvm(kvm);
> +		put_unused_fd(r);
> +		fput(file);
>  		return -ENOMEM;
>  	}
>  
> +	fd_install(r, file);
>  	return r;git
>  }
>  
>