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

[Liu Shuo]

On Thu 14.Jul'16 at 17:46:47 +0100, 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@xxxxxxxxx>
>
> 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
Could you please elaborate why we're not sure descriptor table's changing at the point?
> like that.  Moreover, this kvm_put_kvm() becomes a double-put, since
> closing the damn file will drop that reference to kvm.
You're right. Thanks for the correction on this point.
> > >  		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):
>
> [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@xxxxxxxxxxxxxxxxxx>
Thanks. It's better.
> ---
> 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;
> }