Re: [PATCH net v2] net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()

[Eric Dumazet]

On Mon, Mar 9, 2026 at 3:38 AM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/67eaf9b8.050a0220.3c3d88.004a.GAE@xxxxxxxxxx/T/
> Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> ---
> 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.

This looks better, but please note there is something missing in your
RCU implementation.

You still need to ensure s->sk_user_data is read / written with
load/store tearing prevention.
Standard rcu_dereference()/rcu_assign() are dealing with this aspect.

For instance, the following helper is assuming a lock was held :

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);
}


One possible way is to use
rcu_dereference_sk_user_data()/rcu_assign_sk_user_data() or similar
helpers.


Thanks.