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

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