---
id: CVE-2026-23450
title: 'net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()'
summary: |-
  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 T…
severity: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= ebfee3e153f67c8b38eb94a7062ee94aa6f92708 <
    f315277856caeafcd996c2611afc085ca2d53275
  - >-
    Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e <
    1e4f873879e075bbd4eb1c644d6933303ac5eba4
  - >-
    Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e <
    f00fc26c8a06442b225a350fe000c0a11483e6a3
  - >-
    Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e <
    cadf3da46c15523fba90d80c9955f536ee3b4023
  - >-
    Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e <
    fd7579f0a2c84ba8a7d4f206201b50dc8ddf90c2
  - >-
    Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e <
    1fab5ece76fb42a761178dcd0ebcbf578377b0dd
  - >-
    Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e <
    6d5e4538364b9ceb1ac2941a4deb86650afb3538
  - Linux >= 5.15.174 < 5.15.203
  - Linux 5.18
published: '2026-04-03'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T08:46:17.289Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-23450'
references:
  - url: 'https://git.kernel.org/stable/c/f315277856caeafcd996c2611afc085ca2d53275'
  - url: 'https://git.kernel.org/stable/c/1e4f873879e075bbd4eb1c644d6933303ac5eba4'
  - url: 'https://git.kernel.org/stable/c/f00fc26c8a06442b225a350fe000c0a11483e6a3'
  - url: 'https://git.kernel.org/stable/c/cadf3da46c15523fba90d80c9955f536ee3b4023'
  - url: 'https://git.kernel.org/stable/c/fd7579f0a2c84ba8a7d4f206201b50dc8ddf90c2'
  - url: 'https://git.kernel.org/stable/c/1fab5ece76fb42a761178dcd0ebcbf578377b0dd'
  - url: 'https://git.kernel.org/stable/c/6d5e4538364b9ceb1ac2941a4deb86650afb3538'
tags:
  - cve.org
epss: 0.0056
epssPercentile: 0.44234
ingestedAt: '2026-09-08T15:33:26.992Z'
---

## Overview

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

## Affected

- `Linux >= ebfee3e153f67c8b38eb94a7062ee94aa6f92708 < f315277856caeafcd996c2611afc085ca2d53275`
- `Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e < 1e4f873879e075bbd4eb1c644d6933303ac5eba4`
- `Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e < f00fc26c8a06442b225a350fe000c0a11483e6a3`
- `Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e < cadf3da46c15523fba90d80c9955f536ee3b4023`
- `Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e < fd7579f0a2c84ba8a7d4f206201b50dc8ddf90c2`
- `Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e < 1fab5ece76fb42a761178dcd0ebcbf578377b0dd`
- `Linux >= 8270d9c21041470f58348248b9d9dcf3bf79592e < 6d5e4538364b9ceb1ac2941a4deb86650afb3538`
- `Linux >= 5.15.174 < 5.15.203`
- `Linux 5.18`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.