[PATCH net v3] net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()

Jiayuan Chen
2026-03-10T12:01:24Z

From: Jiayuan Chen <jiayuan.chen@shopee.com>

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 using RCU and refcount_inc_not_zero() to safely
access smc_sock. Since smc_tcp_syn_recv_sock() is called in
the TCP three-way handshake path, taking read_lock_bh on
sk_callback_lock is too heavy and would not survive a SYN
flood attack. Using rcu_read_lock() is much more lightweight.

- Set SOCK_RCU_FREE on the SMC listen socket so that
smc_sock freeing is deferred until after the RCU grace
period. This guarantees the memory is still valid when
accessed inside rcu_read_lock().
- Use rcu_read_lock() to protect reading sk_user_data.
- Use refcount_inc_not_zero(&smc->sk.sk_refcnt) to pin the
smc_sock. If the refcount has already reached zero (close
path completed), it returns false and we bail out safely.

Note: smc_hs_congested() has a similar lockless read of
sk_user_data without rcu_read_lock(), but it only checks for
NULL and accesses the global smc_hs_wq, never dereferencing
any smc_sock field, so it is not affected.

Reproducer was verified with mdelay injection and smc_run,
the issue no longer occurs with this patch applied.

[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/
Suggested-by: Eric Dumazet <edumazet@google.com>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
v2 -> v3:
- Use rcu_dereference_sk_user_data() for reading and
rcu_assign_sk_user_data() for writing sk_user_data to
prevent load/store tearing, as pointed out by Eric Dumazet.
v2: https://lore.kernel.org/netdev/20260309023846.18516-1-jiayuan.chen@linux.dev/

v1 -> v2:
- Use rcu_read_lock() + refcount_inc_not_zero() instead of
read_lock_bh(sk_callback_lock) + sock_hold(), since this
is the TCP handshake hot path and read_lock_bh is too
expensive under SYN flood.
- Set SOCK_RCU_FREE on SMC listen socket to ensure
RCU-deferred freeing.
v1: https://lore.kernel.org/netdev/20260307032158.372165-1-jiayuan.chen@linux.dev/
---
net/smc/af_smc.c | 19 ++++++++++++++-----
net/smc/smc.h | 3 +--
net/smc/smc_close.c | 2 +-
3 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index d0119afcc6a1..078927ba068a 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -131,7 +131,13 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
struct smc_sock *smc;
struct sock *child;

+ rcu_read_lock();
smc = smc_clcsock_user_data(sk);
+ if (!smc || !refcount_inc_not_zero(&smc->sk.sk_refcnt)) {
+ rcu_read_unlock();
+ return NULL;
+ }
+ rcu_read_unlock();

if (READ_ONCE(sk->sk_ack_backlog) + atomic_read(&smc->queued_smc_hs) >
sk->sk_max_ack_backlog)
@@ -153,11 +159,13 @@ 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;
}

@@ -254,7 +262,7 @@ static void smc_fback_restore_callbacks(struct smc_sock *smc)
struct sock *clcsk = smc->clcsock->sk;

write_lock_bh(&clcsk->sk_callback_lock);
- clcsk->sk_user_data = NULL;
+ rcu_assign_sk_user_data(clcsk, NULL);

smc_clcsock_restore_cb(&clcsk->sk_state_change, &smc->clcsk_state_change);
smc_clcsock_restore_cb(&clcsk->sk_data_ready, &smc->clcsk_data_ready);
@@ -902,7 +910,7 @@ static void smc_fback_replace_callbacks(struct smc_sock *smc)
struct sock *clcsk = smc->clcsock->sk;

write_lock_bh(&clcsk->sk_callback_lock);
- clcsk->sk_user_data = (void *)((uintptr_t)smc | SK_USER_DATA_NOCOPY);
+ __rcu_assign_sk_user_data_with_flags(clcsk, smc, SK_USER_DATA_NOCOPY);

smc_clcsock_replace_cb(&clcsk->sk_state_change, smc_fback_state_change,
&smc->clcsk_state_change);
@@ -2665,8 +2673,8 @@ int smc_listen(struct socket *sock, int backlog)
* smc-specific sk_data_ready function
*/
write_lock_bh(&smc->clcsock->sk->sk_callback_lock);
- smc->clcsock->sk->sk_user_data =
- (void *)((uintptr_t)smc | SK_USER_DATA_NOCOPY);
+ __rcu_assign_sk_user_data_with_flags(smc->clcsock->sk, smc,
+ SK_USER_DATA_NOCOPY);
smc_clcsock_replace_cb(&smc->clcsock->sk->sk_data_ready,
smc_clcsock_data_ready, &smc->clcsk_data_ready);
write_unlock_bh(&smc->clcsock->sk->sk_callback_lock);
@@ -2687,10 +2695,11 @@ int smc_listen(struct socket *sock, int backlog)
write_lock_bh(&smc->clcsock->sk->sk_callback_lock);
smc_clcsock_restore_cb(&smc->clcsock->sk->sk_data_ready,
&smc->clcsk_data_ready);
- smc->clcsock->sk->sk_user_data = NULL;
+ rcu_assign_sk_user_data(smc->clcsock->sk, NULL);
write_unlock_bh(&smc->clcsock->sk->sk_callback_lock);
goto out;
}
+ sock_set_flag(sk, SOCK_RCU_FREE);
sk->sk_max_ack_backlog = backlog;
sk->sk_ack_backlog = 0;
sk->sk_state = SMC_LISTEN;
diff --git a/net/smc/smc.h b/net/smc/smc.h
index 9e6af72784ba..8b3eabcdb542 100644
--- a/net/smc/smc.h
+++ b/net/smc/smc.h
@@ -342,8 +342,7 @@ static inline void smc_init_saved_callbacks(struct smc_sock *smc)

static inline struct smc_sock *smc_clcsock_user_data(const struct sock *clcsk)
{
- return (struct smc_sock *)
- ((uintptr_t)clcsk->sk_user_data & ~SK_USER_DATA_NOCOPY);
+ return (struct smc_sock *)rcu_dereference_sk_user_data(clcsk);
}

/* save target_cb in saved_cb, and replace target_cb with new_cb */
diff --git a/net/smc/smc_close.c b/net/smc/smc_close.c
index 10219f55aad1..bb0313ef5f7c 100644
--- a/net/smc/smc_close.c
+++ b/net/smc/smc_close.c
@@ -218,7 +218,7 @@ int smc_close_active(struct smc_sock *smc)
write_lock_bh(&smc->clcsock->sk->sk_callback_lock);
smc_clcsock_restore_cb(&smc->clcsock->sk->sk_data_ready,
&smc->clcsk_data_ready);
- smc->clcsock->sk->sk_user_data = NULL;
+ rcu_assign_sk_user_data(smc->clcsock->sk, NULL);
write_unlock_bh(&smc->clcsock->sk->sk_callback_lock);
rc = kernel_sock_shutdown(smc->clcsock, SHUT_RDWR);
}
--
2.43.0