Could not mount sysfs when enable userns but disable netns

["chenhanxiao@xxxxxxxxxxxxxx" <chenhanxiao@xxxxxxxxxxxxxx>]

Hello,

How to reproduce:
1. Prepare a container, enable userns and disable netns
2. use libvirt-lxc to start a container
3. libvirt could not mount sysfs then failed to start.

Then I found that
commit 7dc5dbc879bd0779924b5132a48b731a0bc04a1e says:
"Don't allow mounting sysfs unless the caller has CAP_SYS_ADMIN rights
over the net namespace."

But why should we check sysfs mouont permission over net namespace?
We've already checked CAP_SYS_ADMIN though.

What the relationship between sysfs and net namespace,
or this check is a little redundant?

Any insights on this?

Thanks,
- Chen

PS: codes below could be a workaround

@@ -34,7 +35,8 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
                if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
                        return ERR_PTR(-EPERM);
 
-               if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
+               if (current->nsproxy->net_ns != &init_net &&
+                    !kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
                        return ERR_PTR(-EPERM);
        }
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