net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()_CVE-2026-23450

{
    "lastseen": "",
    "description": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()\n\nSyzkaller reported a panic in smc_tcp_syn_recv_sock() [1].\n\nsmc_tcp_syn_recv_sock() is called in the TCP receive path\n(softirq) via icsk_af_ops->syn_recv_sock on the clcsock (TCP\nlistening socket). It reads sk_user_data to get the smc_sock\npointer. However, when the SMC listen socket is being closed\nconcurrently, smc_close_active() sets clcsock->sk_user_data\nto NULL under sk_callback_lock, and then the smc_sock itself\ncan be freed via sock_put() in smc_release().\n\nThis leads to two issues:\n\n1) NULL pointer dereference: sk_user_data is NULL when\n   accessed.\n2) Use-after-free: sk_user_data is read as non-NULL, but the\n   smc_sock is freed before its fields (e.g., queued_smc_hs,\n   ori_af_ops) are accessed.\n\nThe race window looks like this (the syzkaller crash [1]\ntriggers via the SYN cookie path: tcp_get_cookie_sock() ->\nsmc_tcp_syn_recv_sock(), but the normal tcp_check_req() path\nhas the same race):\n\n  CPU A (softirq)              CPU B (process ctx)\n\n  tcp_v4_rcv()\n    TCP_NEW_SYN_RECV:\n    sk = req->rsk_listener\n    sock_hold(sk)\n    /* No lock on listener */\n                               smc_close_active():\n                                 write_lock_bh(cb_lock)\n                                 sk_user_data = NULL\n                                 write_unlock_bh(cb_lock)\n                                 ...\n                                 smc_clcsock_release()\n                                 sock_put(smc->sk) x2\n                                   -> smc_sock freed!\n    tcp_check_req()\n      smc_tcp_syn_recv_sock():\n        smc = user_data(sk)\n          -> NULL or dangling\n        smc->queued_smc_hs\n          -> crash!\n\nNote that the clcsock and smc_sock are two independent objects\nwith separate refcounts. TCP stack holds a reference on the\nclcsock, which keeps it alive, but this does NOT prevent the\nsmc_sock from being freed.\n\nFix this by using RCU and refcount_inc_not_zero() to safely\naccess smc_sock. Since smc_tcp_syn_recv_sock() is called in\nthe TCP three-way handshake path, taking read_lock_bh on\nsk_callback_lock is too heavy and would not survive a SYN\nflood attack. Using rcu_read_lock() is much more lightweight.\n\n- Set SOCK_RCU_FREE on the SMC listen socket so that\n  smc_sock freeing is deferred until after the RCU grace\n  period. This guarantees the memory is still valid when\n  accessed inside rcu_read_lock().\n- Use rcu_read_lock() to protect reading sk_user_data.\n- Use refcount_inc_not_zero(&smc->sk.sk_refcnt) to pin the\n  smc_sock. If the refcount has already reached zero (close\n  path completed), it returns false and we bail out safely.\n\nNote: smc_hs_congested() has a similar lockless read of\nsk_user_data without rcu_read_lock(), but it only checks for\nNULL and accesses the global smc_hs_wq, never dereferencing\nany smc_sock field, so it is not affected.\n\nReproducer was verified with mdelay injection and smc_run,\nthe issue no longer occurs with this patch applied.\n\n[1] https://syzkaller.appspot.com/bug?extid=827ae2bfb3a3529333e9",
    "published": "2026-04-03T15:15:33.144Z",
    "modified": "2026-04-27T14:02:29.638Z",
    "type": "cve",
    "title": "net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()",
    "source": "Linux",
    "references": "https://git.kernel.org/stable/c/f315277856caeafcd996c2611afc085ca2d53275\nhttps://git.kernel.org/stable/c/1e4f873879e075bbd4eb1c644d6933303ac5eba4\nhttps://git.kernel.org/stable/c/f00fc26c8a06442b225a350fe000c0a11483e6a3\nhttps://git.kernel.org/stable/c/cadf3da46c15523fba90d80c9955f536ee3b4023\nhttps://git.kernel.org/stable/c/fd7579f0a2c84ba8a7d4f206201b50dc8ddf90c2\nhttps://git.kernel.org/stable/c/1fab5ece76fb42a761178dcd0ebcbf578377b0dd\nhttps://git.kernel.org/stable/c/6d5e4538364b9ceb1ac2941a4deb86650afb3538",
    "id": "CVE-2026-23450",
    "bulletinFamily": "",
    "cwe": null,
    "cvelist": null,
    "sourceData": "Linux Linux ebfee3e153f67c8b38eb94a7062ee94aa6f92708\nLinux Linux 8270d9c21041470f58348248b9d9dcf3bf79592e\nLinux Linux 8270d9c21041470f58348248b9d9dcf3bf79592e\nLinux Linux 8270d9c21041470f58348248b9d9dcf3bf79592e\nLinux Linux 8270d9c21041470f58348248b9d9dcf3bf79592e\nLinux Linux 8270d9c21041470f58348248b9d9dcf3bf79592e\nLinux Linux 8270d9c21041470f58348248b9d9dcf3bf79592e\nLinux Linux 5.18",
    "sourceHref": "",
    "cvss": {
        "score": 9.8,
        "severity": "CRITICAL",
        "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
        "version": "3.1"
    },
    "cvss2": [],
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        "baseScore": 0,
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        "cvssV3": {
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    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "Linux",
    "version": "ebfee3e153f67c8b38eb94a7062ee94aa6f92708",
    "vendor": "Linux",
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    "ai_score": 0
}