[PATCH net v4] ipv6: flowlabel: enforce per-netns limit for unprivileged callers

[Maoyi Xie]

fl_size, fl_ht and ip6_fl_lock in net/ipv6/ip6_flowlabel.c are file
scope and shared across netns. mem_check() reads fl_size to decide
whether to deny non-CAP_NET_ADMIN callers; capable() runs against
init_user_ns, so an unprivileged user in any non-init userns can
push fl_size past FL_MAX_SIZE - FL_MAX_SIZE/4 and starve every
other unprivileged userns on the host.

Add struct netns_ipv6::flowlabel_count, bumped and decremented next
to fl_size in fl_intern, ip6_fl_gc and ip6_fl_purge. The new field
is placed in the existing 4-byte hole after ipmr_seq, so struct
netns_ipv6 stays the same size on 64-bit builds.

mem_check() folds an extra FL_MAX_SIZE/8 ceiling into the existing
non-CAP_NET_ADMIN conditional.

Bump FL_MAX_SIZE from 4096 to 8192. It has been 4096 since the file
was added; machines and connection counts have grown. The new
per-netns ceiling is then 1024 flowlabels, half of FL_MAX_SIZE/4.

CAP_NET_ADMIN against init_user_ns still bypasses both caps.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Suggested-by: Willem de Bruijn <willemb@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@xxxxxxxxxx>
---
v4 (this submission, addressing v3 review by Willem):
    - rephrased the flowlabel_count placement note: dropped the
      flowlabel_has_excl cacheline argument; replaced with the
      simpler "fills the existing 4-byte hole after ipmr_seq" fact.
    - reordered atomic_dec(&...flowlabel_count) to sit immediately
      after atomic_dec(&fl_size) in ip6_fl_gc and ip6_fl_purge so
      the pairing is visually obvious. Both decs now happen before
      fl_free(fl) since fl_free invalidates fl->fl_net. fl_intern
      was already in this order.
v3: addressed Willem's review on the private security@ thread;
    merged FL_MAX_SIZE doubling, dropped test data, moved
    flowlabel_count near ipmr_seq, inlined fl->fl_net in ip6_fl_gc.
v2: per-netns counter + cap, sent to security@ as a 2-patch series.
v1: fix-shape sketch in original disclosure.

 include/net/netns/ipv6.h |  1 +
 net/ipv6/ip6_flowlabel.c | 14 ++++++++++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
index 34bdb1308..329482373 100644
--- a/include/net/netns/ipv6.h
+++ b/include/net/netns/ipv6.h
@@ -119,6 +119,7 @@ struct netns_ipv6 {
 	struct fib_notifier_ops	*notifier_ops;
 	struct fib_notifier_ops	*ip6mr_notifier_ops;
 	unsigned int ipmr_seq; /* protected by rtnl_mutex */
+	atomic_t		flowlabel_count;
 	struct {
 		struct hlist_head head;
 		spinlock_t	lock;
diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
index c92f98c6f..360109cad 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c
@@ -36,7 +36,7 @@
 /* FL hash table */
 
 #define FL_MAX_PER_SOCK	32
-#define FL_MAX_SIZE	4096
+#define FL_MAX_SIZE	8192
 #define FL_HASH_MASK	255
 #define FL_HASH(l)	(ntohl(l)&FL_HASH_MASK)
 
@@ -162,8 +162,9 @@ static void ip6_fl_gc(struct timer_list *unused)
 				ttd = fl->expires;
 				if (time_after_eq(now, ttd)) {
 					*flp = fl->next;
-					fl_free(fl);
 					atomic_dec(&fl_size);
+					atomic_dec(&fl->fl_net->ipv6.flowlabel_count);
+					fl_free(fl);
 					continue;
 				}
 				if (!sched || time_before(ttd, sched))
@@ -195,8 +196,9 @@ static void __net_exit ip6_fl_purge(struct net *net)
 			if (net_eq(fl->fl_net, net) &&
 			    atomic_read(&fl->users) == 0) {
 				*flp = fl->next;
-				fl_free(fl);
 				atomic_dec(&fl_size);
+				atomic_dec(&net->ipv6.flowlabel_count);
+				fl_free(fl);
 				continue;
 			}
 			flp = &fl->next;
@@ -245,6 +247,7 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 	fl->next = fl_ht[FL_HASH(fl->label)];
 	rcu_assign_pointer(fl_ht[FL_HASH(fl->label)], fl);
 	atomic_inc(&fl_size);
+	atomic_inc(&net->ipv6.flowlabel_count);
 	spin_unlock_bh(&ip6_fl_lock);
 	rcu_read_unlock();
 	return NULL;
@@ -464,6 +467,7 @@ fl_create(struct net *net, struct sock *sk, struct in6_flowlabel_req *freq,
 
 static int mem_check(struct sock *sk)
 {
+	struct net *net = sock_net(sk);
 	int room = FL_MAX_SIZE - atomic_read(&fl_size);
 	struct ipv6_fl_socklist *sfl;
 	int count = 0;
@@ -478,7 +482,9 @@ static int mem_check(struct sock *sk)
 
 	if (room <= 0 ||
 	    ((count >= FL_MAX_PER_SOCK ||
-	      (count > 0 && room < FL_MAX_SIZE/2) || room < FL_MAX_SIZE/4) &&
+	      (count > 0 && room < FL_MAX_SIZE/2) ||
+	      room < FL_MAX_SIZE/4 ||
+	      atomic_read(&net->ipv6.flowlabel_count) >= FL_MAX_SIZE/8) &&
 	     !capable(CAP_NET_ADMIN)))
 		return -ENOBUFS;
 
-- 
2.34.1