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

[Liu Shuo <shuo.a.liu@intel.com>]

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@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
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@zeniv.linux.org.uk>
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;
> }
>