Re: [PATCH net-next 2/2 v3] netns: restrict uevents

[Christian Brauner <christian.brauner@canonical.com>]

On Fri, Apr 27, 2018 at 11:30:26AM -0500, Eric W. Biederman wrote:
> Christian Brauner <christian.brauner@ubuntu.com> writes:
> > ---
> >  lib/kobject_uevent.c | 140 ++++++++++++++++++++++++++++++-------------
> >  1 file changed, 99 insertions(+), 41 deletions(-)
> >
> > diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> > index c3cb110f663b..d8ce5e6d83af 100644
> > --- a/lib/kobject_uevent.c
> > +++ b/lib/kobject_uevent.c
> >  
> > +static int uevent_net_broadcast_tagged(struct sock *usk,
> > +				       struct kobj_uevent_env *env,
> > +				       const char *action_string,
> > +				       const char *devpath)
> > +{
> > +	struct user_namespace *owning_user_ns = sock_net(usk)->user_ns;
> > +	struct sk_buff *skb = NULL;
> > +	int ret;
> > +
> > +	skb = alloc_uevent_skb(env, action_string, devpath);
> > +	if (!skb)
> > +		return -ENOMEM;
> > +
> > +	/* fix credentials */
> > +	if (owning_user_ns != &init_user_ns) {
> 
> Nit: This test is just a performance optimization as such is not
>       necessary.  That is we can safely unconditionally set the
>       credentials this way.

alloc_uevent_skb() will now set

	parms = &NETLINK_CB(skb);
	parms->creds.uid = GLOBAL_ROOT_UID;
	parms->creds.gid = GLOBAL_ROOT_GID;
	parms->dst_group = 1;
	parms->portid = 0;

explicitly. So repeating that initialization unconditionally here does
not make sense to me. Also, this hits map_uid_down() in user_namespace.c
which is a known-hotpath (Remember the extensive testing we did back for
uidmap limit bumping from 5 to 340.). And even though it might not
matter much in this case there's no need to hit this code. The condition
also make it obvious that only non-initial user namespace uevent sockets
need fixing.

Christian

> 
> > +		struct netlink_skb_parms *parms = &NETLINK_CB(skb);
> > +		kuid_t root_uid;
> > +		kgid_t root_gid;
> > +
> > +		/* fix uid */
> > +		root_uid = make_kuid(owning_user_ns, 0);
> > +		if (!uid_valid(root_uid))
> > +			root_uid = GLOBAL_ROOT_UID;
> > +		parms->creds.uid = root_uid;
> > +
> > +		/* fix gid */
> > +		root_gid = make_kgid(owning_user_ns, 0);
> > +		if (!gid_valid(root_gid))
> > +			root_gid = GLOBAL_ROOT_GID;
> > +		parms->creds.gid = root_gid;
> > +	}
> > +
> > +	ret = netlink_broadcast(usk, skb, 0, 1, GFP_KERNEL);
> > +	/* ENOBUFS should be handled in userspace */
> > +	if (ret == -ENOBUFS || ret == -ESRCH)
> > +		ret = 0;
> > +
> > +	return ret;
> > +}
> > +#endif