netdev - Re: [PATCH 7/9] net: centralize ns

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Message-ID: <20250919-fanatiker-ethik-7a9bb32ee334@brauner>
Date: Fri, 19 Sep 2025 10:08:33 +0200
From: Christian Brauner <brauner@...nel.org>
To: Jan Kara <jack@...e.cz>
Cc: linux-fsdevel@...r.kernel.org, Amir Goldstein <amir73il@...il.com>, 
	Josef Bacik <josef@...icpanda.com>, Jeff Layton <jlayton@...nel.org>, Mike Yuan <me@...dnzj.com>, 
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, Lennart Poettering <mzxreary@...inter.de>, 
	Daan De Meyer <daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Koutný <mkoutny@...e.com>, Jakub Kicinski <kuba@...nel.org>, 
	Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH 7/9] net: centralize ns_common initialization

On Thu, Sep 18, 2025 at 11:42:38AM +0200, Jan Kara wrote:
> On Wed 17-09-25 12:28:06, Christian Brauner wrote:
> > Centralize ns_common initialization.
> > 
> > Signed-off-by: Christian Brauner <brauner@...nel.org>
> > ---
> >  net/core/net_namespace.c | 23 +++--------------------
> >  1 file changed, 3 insertions(+), 20 deletions(-)
> > 
> > diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> > index a57b3cda8dbc..897f4927df9e 100644
> > --- a/net/core/net_namespace.c
> > +++ b/net/core/net_namespace.c
> > @@ -409,7 +409,7 @@ static __net_init int preinit_net(struct net *net, struct user_namespace *user_n
> >  	ns_ops = NULL;
> >  #endif
> >  
> > -	ret = ns_common_init(&net->ns, ns_ops, false);
> > +	ret = ns_common_init(&net->ns, ns_ops, true);
> >  	if (ret)
> >  		return ret;
> >  
> > @@ -597,6 +597,7 @@ struct net *copy_net_ns(unsigned long flags,
> >  		net_passive_dec(net);
> >  dec_ucounts:
> >  		dec_net_namespaces(ucounts);
> > +		ns_free_inum(&net->ns);
> 
> This looks like a wrong place to put it? dec_ucounts also gets called when we
> failed to create 'net' and thus net == NULL. 
> 
> >  		return ERR_PTR(rv);
> >  	}
> >  	return net;
> > @@ -718,6 +719,7 @@ static void cleanup_net(struct work_struct *work)
> >  #endif
> >  		put_user_ns(net->user_ns);
> >  		net_passive_dec(net);
> > +		ns_free_inum(&net->ns);
> 
> The calling of ns_free_inum() after we've dropped our reference
> (net_passive_dec()) looks suspicious. Given how 'net' freeing works I don't
> think this can lead to actual UAF issues but it is in my opinion a bad
> coding pattern and for no good reason AFAICT.

All good points. I can't say I'm fond of the complexity in this specific
instance in general.

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 897f4927df9e..9df236811454 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -590,6 +590,7 @@ struct net *copy_net_ns(unsigned long flags,

        if (rv < 0) {
 put_userns:
+               ns_free_inum(&net->ns);
 #ifdef CONFIG_KEYS
                key_remove_domain(net->key_domain);
 #endif
@@ -597,7 +598,6 @@ struct net *copy_net_ns(unsigned long flags,
                net_passive_dec(net);
 dec_ucounts:
                dec_net_namespaces(ucounts);
-               ns_free_inum(&net->ns);
                return ERR_PTR(rv);
        }
        return net;
@@ -713,13 +713,13 @@ static void cleanup_net(struct work_struct *work)
        /* Finally it is safe to free my network namespace structure */
        list_for_each_entry_safe(net, tmp, &net_exit_list, exit_list) {
                list_del_init(&net->exit_list);
+               ns_free_inum(&net->ns);
                dec_net_namespaces(net->ucounts);
 #ifdef CONFIG_KEYS
                key_remove_domain(net->key_domain);
 #endif
                put_user_ns(net->user_ns);
                net_passive_dec(net);
-               ns_free_inum(&net->ns);
        }
        WRITE_ONCE(cleanup_net_task, NULL);
 }