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

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