[PATCH net v1] net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()

Jiayuan Chen
2026-03-07T03:22:12Z

Syzkaller reported a panic in smc_tcp_syn_recv_sock() [1].

smc_tcp_syn_recv_sock() is called in the TCP receive path
(softirq) via icsk_af_ops->syn_recv_sock on the clcsock (TCP
listening socket). It reads sk_user_data to get the smc_sock
pointer. However, when the SMC listen socket is being closed
concurrently, smc_close_active() sets clcsock->sk_user_data
to NULL under sk_callback_lock, and then the smc_sock itself
can be freed via sock_put() in smc_release().

This leads to two issues:

1) NULL pointer dereference: sk_user_data is NULL when
accessed.
2) Use-after-free: sk_user_data is read as non-NULL, but the
smc_sock is freed before its fields (e.g., queued_smc_hs,
ori_af_ops) are accessed.

The race window looks like this:

CPU A (softirq) CPU B (process ctx)

tcp_v4_rcv()
TCP_NEW_SYN_RECV:
sk = req->rsk_listener
sock_hold(sk)
/* No lock on listener */
smc_close_active():
write_lock_bh(cb_lock)
sk_user_data = NULL
write_unlock_bh(cb_lock)
...
smc_clcsock_release()
sock_put(smc->sk) x2
-> smc_sock freed!
tcp_check_req()
smc_tcp_syn_recv_sock():
smc = user_data(sk)
-> NULL or dangling
smc->queued_smc_hs
-> crash!

Note that the clcsock and smc_sock are two independent objects
with separate refcounts. TCP stack holds a reference on the
clcsock, which keeps it alive, but this does NOT prevent the
smc_sock from being freed.

Fix this by taking sk_callback_lock to read sk_user_data and
then sock_hold(&smc->sk) under the lock to pin the smc_sock.
The lock is released immediately after sock_hold(), rather
than being held for the entire function, to avoid holding it
across ori_af_ops->syn_recv_sock() which creates child
sockets and could risk deadlocks with nested lock ordering.
sock_put(&smc->sk) is called on all exit paths after the
hold.

[1] https://syzkaller.appspot.com/bug?extid=827ae2bfb3a3529333e9

Fixes: 8270d9c21041 ("net/smc: Limit backlog connections")
Reported-by: syzbot+827ae2bfb3a3529333e9@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67eaf9b8.050a0220.3c3d88.004a.GAE@google.com/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
net/smc/af_smc.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index d0119afcc6a1..21218b9b0f9a 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -131,7 +131,14 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
struct smc_sock *smc;
struct sock *child;

+ read_lock_bh(&((struct sock *)sk)->sk_callback_lock);
smc = smc_clcsock_user_data(sk);
+ if (!smc) {
+ read_unlock_bh(&((struct sock *)sk)->sk_callback_lock);
+ return NULL;
+ }
+ sock_hold(&smc->sk);
+ read_unlock_bh(&((struct sock *)sk)->sk_callback_lock);

if (READ_ONCE(sk->sk_ack_backlog) + atomic_read(&smc->queued_smc_hs) >
sk->sk_max_ack_backlog)
@@ -153,11 +160,13 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
if (inet_csk(child)->icsk_af_ops == inet_csk(sk)->icsk_af_ops)
inet_csk(child)->icsk_af_ops = smc->ori_af_ops;
}
+ sock_put(&smc->sk);
return child;

drop:
dst_release(dst);
tcp_listendrop(sk);
+ sock_put(&smc->sk);
return NULL;
}

--
2.43.0