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:
__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
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()"
The fix is applied locally in L2TP’s IPv6 transmit path before calling
inet6_csk_xmit(). First, it performs a lockless pre-validation of the
socket route cache via sk_dst_check(), so that any obsolete cached dst
is atomically removed from sk->sk_dst_cache by the lockless helper
(through its xchg() path); this prevents the locked __sk_dst_check()
inside inet6_csk_xmit() from issuing a second dst_release() on the same
cache-owned reference. Second, it takes an additional reference to the
current cached dst with sk_dst_get() and drops it after inet6_csk_xmit()
returns, ensuring the dst lifetime is guarded while L2TP transmits, even
if the cache is concurrently updated. Together these steps eliminate the
double-release race without changing sock-core semantics.
Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Fixes: d14730b8e911 ("ipv6: use RCU in inet6_csk_xmit()")
Signed-off-by: Mikhail Lobanov
---
v2: move fix to L2TP as suggested by Eric Dumazet.
net/l2tp/l2tp_core.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 369a2f2e459c..93dafac9117f 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1210,9 +1210,17 @@ static int l2tp_xmit_queue(struct l2tp_tunnel *tunnel, struct sk_buff *skb, stru
skb->ignore_df = 1;
skb_dst_drop(skb);
#if IS_ENABLED(CONFIG_IPV6)
- if (l2tp_sk_is_v6(tunnel->sock))
+ if (l2tp_sk_is_v6(tunnel->sock)) {
+ struct dst_entry *pre_dst, *hold_dst;
+
+ pre_dst = sk_dst_check(tunnel->sock, 0);
+ if (pre_dst)
+ dst_release(pre_dst);
+ hold_dst = sk_dst_get(tunnel->sock);
err = inet6_csk_xmit(tunnel->sock, skb, NULL);
- else
+ if (hold_dst)
+ dst_release(hold_dst);
+ } else
#endif
err = ip_queue_xmit(tunnel->sock, skb, fl);
--
2.47.2