ipv6: per-netns exclusive flowlabel checks

Ipv6 flowlabels historically require a reservation before use.
Optionally in exclusive mode (e.g., user-private).

Commit 59c820b ("ipv6: elide flowlabel check if no exclusive
leases exist") introduced a fastpath that avoids this check when no
exclusive leases exist in the system, and thus any flowlabel use
will be granted.

That allows skipping the control operation to reserve a flowlabel
entirely. Though with a warning if the fast path fails:

  This is an optimization. Robust applications still have to revert to
  requesting leases if the fast path fails due to an exclusive lease.

Still, this is subtle. Better isolate network namespaces from each
other. Flowlabels are per-netns. Also record per-netns whether
exclusive leases are in use. Then behavior does not change based on
activity in other netns.

Fixes: 59c820b ("ipv6: elide flowlabel check if no exclusive leases exist")
Link: https://lore.kernel.org/netdev/MWHPR2201MB1072BCCCFCE779E4094837ACD0329@MWHPR2201MB1072.namprd22.prod.outlook.com/
Reported-by: Congyu Liu <liu3101@purdue.edu>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Tested-by: Congyu Liu <liu3101@purdue.edu>

include/net/ipv6.h

@@ -399,7 +399,8 @@ extern struct static_key_false_deferred ipv6_flowlabel_exclusive;
 static inline struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk,
 						    __be32 label)
 {
-	if (static_branch_unlikely(&ipv6_flowlabel_exclusive.key))
+	if (static_branch_unlikely(&ipv6_flowlabel_exclusive.key) &&
+	    READ_ONCE(sock_net(sk)->ipv6.flowlabel_has_excl))
 		return __fl6_sock_lookup(sk, label) ? : ERR_PTR(-ENOENT);
 
 	return NULL;

include/net/netns/ipv6.h

@@ -77,9 +77,10 @@ struct netns_ipv6 {
 	spinlock_t		fib6_gc_lock;
 	unsigned int		 ip6_rt_gc_expire;
 	unsigned long		 ip6_rt_last_gc;
+	unsigned char		flowlabel_has_excl;
 #ifdef CONFIG_IPV6_MULTIPLE_TABLES
-	unsigned int		fib6_rules_require_fldissect;
 	bool			fib6_has_custom_rules;
+	unsigned int		fib6_rules_require_fldissect;
 #ifdef CONFIG_IPV6_SUBTREES
 	unsigned int		fib6_routes_require_src;
 #endif

net/ipv6/ip6_flowlabel.c

@@ -450,8 +450,10 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
 		err = -EINVAL;
 		goto done;
 	}
-	if (fl_shared_exclusive(fl) || fl->opt)
+	if (fl_shared_exclusive(fl) || fl->opt) {
+		WRITE_ONCE(sock_net(sk)->ipv6.flowlabel_has_excl, 1);
 		static_branch_deferred_inc(&ipv6_flowlabel_exclusive);
+	}
 	return fl;
 
 done: