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

From: Liu Shuo
Date: Thu Jul 14 2016 - 22:22:00 EST

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

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;
- r = anon_inode_getfd("kvm-vm", &kvm_vm_fops, kvm, O_RDWR | O_CLOEXEC);
+ r = get_unused_fd_flags(O_CLOEXEC);
if (r < 0) {
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;