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SubjectRe: [PATCH] KVM: release anon file in failure path of vm creation
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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
> }
>
>

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