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....@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 <v...@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;
}