[PATCH net v3 1/1] ipv6: flowlabel: cap duplicate leases per socket

From: Zhiling Zou <redacted>

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 the socket's existing leases during the lookup and reject a new
unprivileged lease once the count reaches FL_MAX_PER_SOCK. This matches
mem_check()'s per-socket accounting and also covers leases acquired from
distinct globally interned labels. The IPv6 sockopt lock serializes the
count with fl_link() and PUT, so no global ip6_fl_lock is needed.

Check CAP_NET_ADMIN only when the socket reaches the limit. This avoids a
capability audit on successful unprivileged GET requests below the cap,
while privileged callers remain unrestricted.

Do the admission check before updating linger and expires. A rejected GET
does not refresh 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 v3:
- Count the socket's full lease list instead of only matching leases.
- Defer the CAP_NET_ADMIN check until the lease count reaches the limit.
- v2 Link: https://lore.kernel.org/all/528bc30d301bcf32aca37cda1933b617d2d295f4.1786447968.git.zhilinz@nebusec.ai/ (local)

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..b864dfc87f779 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c

@@ -617,6 +617,7 @@ 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);
+	int lease_count = 0;
 	int err;
 
 	if (freq->flr_flags & IPV6_FL_F_REFLECT) {

@@ -647,16 +648,20 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq,
 		err = -EEXIST;
 		rcu_read_lock();
 		for_each_sk_fl_rcu(sk, sfl) {
+			lease_count++;
 			if (sfl->fl->label == freq->flr_label) {
 				if (freq->flr_flags & IPV6_FL_F_EXCL) {
 					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;
+				}
 			}
+			if (fl1 && lease_count >= FL_MAX_PER_SOCK)
+				break;
 		}
 		rcu_read_unlock();
 

@@ -679,6 +684,11 @@ 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 (lease_count >= FL_MAX_PER_SOCK &&
+			    !capable(CAP_NET_ADMIN))
+				goto release;
 			if (fl->linger > fl1->linger)
 				fl1->linger = fl->linger;
 			if ((long)(fl->expires - fl1->expires) > 0)

-- 
2.43.0