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

Jiayuan Chen
2026-03-10T12:38:41Z

On 3/10/26 8:13 PM, Eric Dumazet wrote:
> On Tue, Mar 10, 2026 at 1:01 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>> 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!
>>
>
>
>> 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);
>> }
> Are you sure all smc_clcsock_user_data() callers hold rcu_read_lock() ?
> In order to avoid surprises, I would have added a new helper.
>
> static inline struct smc_sock *smc_clcsock_user_data_rcu(const struct
> sock *clcsk)
> ...
>
> to allow gradual conversion ?
>
> Thanks !

Sorry I missed that.

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