net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()

In the Linux kernel, the following vulnerability has been resolved:

net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()

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 (the syzkaller crash [1]
triggers via the SYN cookie path: tcp_get_cookie_sock() ->
smc_tcp_syn_recv_sock(), but the normal tcp_check_req() path
has the same race):

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