Re: Could not mount sysfs when enable userns but disable netns

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting chenhanxiao@cn.fujitsu.com (chenhanxiao@cn.fujitsu.com):
> 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?

It is not redundant.  The whole point is that after clone(CLONE_NEWUSER)
you get a newly filled set of capabilities.  But you should not have
privileges over the host's network namesapce.  After you unshare a new
network namespace, you *should* have privilege over it.  So the fact
that we've already check CAP_SYS_ADMIN means nothing, because the
capabilities need to be targeted.

> 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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