[PATCH net v2 1/1] ipv6: flowlabel: cap duplicate leases per socket

ipv6_flowlabel_get() allocates an ipv6_fl_socklist entry for every
successful GET. The recheck path for a compatible existing flowlabel
links another lease without applying any lease admission check. Repeated
GET requests for one shareable label can therefore grow a socket's lease
list without bound.

Count matching leases during the existing socket-list lookup and reject
a new unprivileged lease once that count reaches FL_MAX_PER_SOCK. The
IPv6 sockopt lock serializes the count with fl_link() and PUT, so no
global ip6_fl_lock is needed. New-label admission remains under the
existing mem_check() policy. CAP_NET_ADMIN callers remain unrestricted
and stop after the first matching lease.

Do the check before updating linger and expires. A GET that cannot add
its lease returns an error without refreshing the shared label, matching
the existing socket-list allocation failure path.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <redacted>
Signed-off-by: Zhiling Zou <redacted>
---
changes in v2:
- Count only leases of the flowlabel being reused.
- Fold the count into the existing socket-list lookup.
- Keep new-label admission under the existing mem_check() policy.
- Stop after the first matching lease for CAP_NET_ADMIN callers.
- Explain why a rejected GET does not refresh linger or expires.
- Correct the reuse-path explanation and trim the Fixes hash.
- v1 Link: https://lore.kernel.org/all/cf4fdc79ae4dc46bd4eb7eeb57e5de2091c13cd3.1785746178.git.zhilinz@nebusec.ai/ (local)

 net/ipv6/ip6_flowlabel.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
index 1ab5ad0dcf24f..55cc5ec485c59 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c

@@ -617,6 +617,8 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq,
 	struct ipv6_fl_socklist *sfl, *sfl1 = NULL;
 	struct ip6_flowlabel *fl, *fl1 = NULL;
 	struct net *net = sock_net(sk);
+	bool cap_net_admin = false;
+	int dup_count = 0;
 	int err;
 
 	if (freq->flr_flags & IPV6_FL_F_REFLECT) {

@@ -652,10 +654,14 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq,
 					rcu_read_unlock();
 					goto done;
 				}
-				fl1 = sfl->fl;
-				if (!atomic_inc_not_zero(&fl1->users))
-					fl1 = NULL;
-				break;
+				if (!fl1) {
+					fl1 = sfl->fl;
+					if (!atomic_inc_not_zero(&fl1->users))
+						fl1 = NULL;
+					cap_net_admin = capable(CAP_NET_ADMIN);
+				}
+				if (++dup_count >= FL_MAX_PER_SOCK || cap_net_admin)
+					break;
 			}
 		}
 		rcu_read_unlock();

@@ -679,6 +685,10 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq,
 			err = -ENOMEM;
 			if (!sfl1)
 				goto release;
+			/* sockopt_lock_sock() serializes the count and fl_link(). */
+			err = -ENOBUFS;
+			if (dup_count >= FL_MAX_PER_SOCK && !cap_net_admin)
+				goto release;
 			if (fl->linger > fl1->linger)
 				fl1->linger = fl->linger;
 			if ((long)(fl->expires - fl1->expires) > 0)

-- 
2.43.0