net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock()

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.