[PATCH net v3] ipv6: flowlabel: enforce per-netns limit for unprivileged callers

[Maoyi Xie <maoyixie.tju@gmail.com>]

From: Maoyi Xie <maoyi.xie@ntu.edu.sg>

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. Place it near
ipmr_seq rather than next to flowlabel_has_excl: flowlabel_has_excl
is read on every flowlabel lookup, and a counter written on every
alloc would dirty its cacheline.

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@google.com>
Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
---
v3 (this submission, netdev): addressed Willem's review on the
    private security@ thread:
    - merged the FL_MAX_SIZE doubling into this patch
    - dropped the test data block from the commit body
    - moved flowlabel_count to a 4-byte hole next to ipmr_seq, off
      the flowlabel_has_excl cacheline
    - inlined fl->fl_net in ip6_fl_gc (no local var)
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 | 10 ++++++++--
 2 files changed, 9 insertions(+), 2 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..4a5219356 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)
 
@@ -162,6 +162,7 @@ static void ip6_fl_gc(struct timer_list *unused)
 				ttd = fl->expires;
 				if (time_after_eq(now, ttd)) {
 					*flp = fl->next;
+					atomic_dec(&fl->fl_net->ipv6.flowlabel_count);
 					fl_free(fl);
 					atomic_dec(&fl_size);
 					continue;
@@ -195,6 +196,7 @@ 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;
+				atomic_dec(&net->ipv6.flowlabel_count);
 				fl_free(fl);
 				atomic_dec(&fl_size);
 				continue;
@@ -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