hulk inclusion category: bugfix bugzilla: https://atomgit.com/openeuler/kernel/issues/8696 Reference: https://lore.kernel.org/netdev/20251215145537.5085-1-m.lobanov@rosa.ru/ ---------------------------------------- A reproducible rcuref - imbalanced put() warning is observed under IPv6 L2TP (pppol2tp) traffic with blackhole routes, indicating an imbalance in dst reference counting for routes cached in sk->sk_dst_cache and pointing to a subtle lifetime/synchronization issue between the helpers that validate and drop cached dst entries. rcuref - imbalanced put() WARNING: CPU: 0 PID: 899 at lib/rcuref.c:266 rcuref_put_slowpath+0x1ce/0x240 lib/rcuref.c:266 Modules linked in: CPSocket connected tcp:127.0.0.1:48148,server=on <-> 127.0.0.1:33750 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:rcuref_put_slowpath+0x1ce/0x240 lib/rcuref.c:266 Call Trace: <TASK> __rcuref_put include/linux/rcuref.h:97 [inline] rcuref_put include/linux/rcuref.h:153 [inline] dst_release+0x291/0x310 net/core/dst.c:167 __sk_dst_check+0x2d4/0x350 net/core/sock.c:604 __inet6_csk_dst_check net/ipv6/inet6_connection_sock.c:76 [inline] inet6_csk_route_socket+0x6ed/0x10c0 net/ipv6/inet6_connection_sock.c:104 inet6_csk_xmit+0x12f/0x740 net/ipv6/inet6_connection_sock.c:121 l2tp_xmit_queue net/l2tp/l2tp_core.c:1214 [inline] l2tp_xmit_core net/l2tp/l2tp_core.c:1309 [inline] l2tp_xmit_skb+0x1404/0x1910 net/l2tp/l2tp_core.c:1325 pppol2tp_sendmsg+0x3ca/0x550 net/l2tp/l2tp_ppp.c:302 sock_sendmsg_nosec net/socket.c:729 [inline] __sock_sendmsg net/socket.c:744 [inline] ____sys_sendmsg+0xab2/0xc70 net/socket.c:2609 ___sys_sendmsg+0x11d/0x1c0 net/socket.c:2663 __sys_sendmmsg+0x188/0x450 net/socket.c:2749 __do_sys_sendmmsg net/socket.c:2778 [inline] __se_sys_sendmmsg net/socket.c:2775 [inline] __x64_sys_sendmmsg+0x98/0x100 net/socket.c:2775 do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0x64/0x140 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x76/0x7e RIP: 0033:0x7fe6960ec719 </TASK> The race occurs between the lockless UDPv6 transmit path (udpv6_sendmsg() -> sk_dst_check()) and the locked L2TP/pppol2tp transmit path (pppol2tp_sendmsg() -> l2tp_xmit_skb() -> ... -> inet6_csk_xmit() → __sk_dst_check()), when both handle the same obsolete dst from sk->sk_dst_cache: the UDPv6 side takes an extra reference and atomically steals and releases the cached dst, while the L2TP side, using a stale cached pointer, still calls dst_release() on it, and together these updates produce an extra final dst_release() on that dst, triggering rcuref - imbalanced put(). The Race Condition: Initial: sk->sk_dst_cache = dst ref(dst) = 1 Thread 1: sk_dst_check() Thread 2: __sk_dst_check() ------------------------ ---------------------------- sk_dst_get(sk): rcu_read_lock() dst = rcu_dereference(sk->sk_dst_cache) rcuref_get(dst) succeeds rcu_read_unlock() // ref = 2 dst = __sk_dst_get(sk) // reads same dst from sk_dst_cache // ref still = 2 (no extra get) [both see dst obsolete & check() == NULL] sk_dst_reset(sk): old = xchg(&sk->sk_dst_cache, NULL) // old = dst dst_release(old) // drop cached ref // ref: 2 -> 1 RCU_INIT_POINTER(sk->sk_dst_cache, NULL) // cache already NULL after xchg dst_release(dst) // ref: 1 -> 0 dst_release(dst) // tries to drop its own ref after final put // rcuref_put_slowpath() -> "rcuref - imbalanced put()" Make L2TP's IPv6 transmit path stop using inet6_csk_xmit() (and thus __sk_dst_check()) and instead open-code the same routing and transmit sequence using the existing sk_dst_check() with dst reference counting and ip6_dst_lookup_flow(). The new code builds a flowi6 from the socket fields in the same way as inet6_csk_route_socket(), then performs dst lookup using sk_dst_check() with proper reference counting, attaches the resulting dst to the skb via skb_dst_set(), and finally invokes ip6_xmit() for transmission. This makes both the UDPv6 and L2TP IPv6 paths use the same dst-cache handling logic for a given socket and removes the possibility that the lockless sk_dst_check() and the locked __sk_dst_check() concurrently drop the same cached dst and trigger the rcuref - imbalanced put() warning under concurrent traffic. Found by Linux Verification Center (linuxtesting.org) with Syzkaller. Fixes: b0270e91014d ("ipv4: add a sock pointer to ip_queue_xmit()") Signed-off-by: Li Xiasong <lixiasong1@huawei.com> --- net/l2tp/l2tp_core.c | 60 +++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 57 insertions(+), 3 deletions(-) diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index f85b383811d5..79a5f635b049 100644 --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -1000,19 +1000,73 @@ static int l2tp_build_l2tpv3_header(struct l2tp_session *session, void *buf) return bufp - optr; } +#if IS_ENABLED(CONFIG_IPV6) +static int l2tp_xmit_ipv6(struct sock *sk, struct sk_buff *skb) +{ + struct ipv6_pinfo *np = inet6_sk(sk); + struct inet_sock *inet = inet_sk(sk); + struct in6_addr *final_p, final; + struct dst_entry *dst; + struct flowi6 fl6; + int err; + + memset(&fl6, 0, sizeof(fl6)); + fl6.flowi6_proto = sk->sk_protocol; + fl6.daddr = sk->sk_v6_daddr; + fl6.saddr = np->saddr; + fl6.flowlabel = np->flow_label; + IP6_ECN_flow_xmit(sk, fl6.flowlabel); + + fl6.flowi6_oif = sk->sk_bound_dev_if; + fl6.flowi6_mark = sk->sk_mark; + fl6.fl6_sport = inet->inet_sport; + fl6.fl6_dport = inet->inet_dport; + fl6.flowi6_uid = sk->sk_uid; + + security_sk_classify_flow(sk, flowi6_to_flowi_common(&fl6)); + + rcu_read_lock(); + final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final); + rcu_read_unlock(); + + dst = sk_dst_check(sk, np->dst_cookie); + if (!dst) { + dst = ip6_dst_lookup_flow(sock_net(sk), sk, &fl6, final_p); + if (IS_ERR(dst)) { + WRITE_ONCE(sk->sk_err_soft, -PTR_ERR(dst)); + sk->sk_route_caps = 0; + kfree_skb(skb); + return PTR_ERR(dst); + } + + ip6_dst_store(sk, dst_clone(dst), NULL, NULL); + } + + skb_dst_set(skb, dst); + fl6.daddr = sk->sk_v6_daddr; + + rcu_read_lock(); + err = ip6_xmit(sk, skb, &fl6, sk->sk_mark, rcu_dereference(np->opt), + np->tclass, sk->sk_priority); + rcu_read_unlock(); + return err; +} +#endif + /* Queue the packet to IP for output: tunnel socket lock must be held */ static int l2tp_xmit_queue(struct l2tp_tunnel *tunnel, struct sk_buff *skb, struct flowi *fl) { int err; + struct sock *sk = tunnel->sock; skb->ignore_df = 1; skb_dst_drop(skb); #if IS_ENABLED(CONFIG_IPV6) - if (l2tp_sk_is_v6(tunnel->sock)) - err = inet6_csk_xmit(tunnel->sock, skb, NULL); + if (l2tp_sk_is_v6(sk)) + err = l2tp_xmit_ipv6(sk, skb); else #endif - err = ip_queue_xmit(tunnel->sock, skb, fl); + err = ip_queue_xmit(sk, skb, fl); return err >= 0 ? NET_XMIT_SUCCESS : NET_XMIT_DROP; } -- 2.34.1