Linux kernel net/smc smc_tcp_syn_recv_sock() NULL dereference and use-after-free
CVE-2026-23450 is a race-condition-driven memory-safety vulnerability in the Linux kernel net/smc subsystem, specifically in smc_tcp_syn_recv_sock(). The function is invoked in the TCP receive path via icsk_af_ops->syn_recv_sock on the clcsock TCP listening socket and reads clcsock->sk_user_data to obtain the associated smc_sock pointer. During concurrent closure of the SMC listen socket, smc_close_active() can clear clcsock->sk_user_data to NULL under sk_callback_lock, and the referenced smc_sock can then be freed via sock_put() in smc_release(). Because clcsock and smc_sock are separate objects with independent reference counts, a reference held by the TCP stack on clcsock does not keep smc_sock alive. As a result, smc_tcp_syn_recv_sock() may either dereference a NULL sk_user_data pointer or dereference a dangling smc_sock pointer after free when accessing fields such as queued_smc_hs or ori_af_ops. The issue was reported by syzkaller and is reachable through both the SYN cookie path tcp_get_cookie_sock() -> smc_tcp_syn_recv_sock() and the normal tcp_check_req() path. The upstream fix protects sk_user_data access with RCU, defers freeing with SOCK_RCU_FREE, and uses refcount_inc_not_zero(&smc->sk.sk_refcnt) to safely pin smc_sock before dereference.
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