Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A22AC241695; Mon, 23 Mar 2026 14:49:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774277349; cv=none; b=koru4805lwyUSHsJQqyi7C9k8zYFFmedKTs8n6grd+w60e32dQKzCmTkYwjYeh7vII87N/EKR8cTskwsyI0CyBGJbvtAWbjY6as7mfsWfPjMUd/RM/BEo9dXrrmNBgs74Vf0u8Ax7lyMZvLA83nqfhckh/rhz+7K2B7AfqF3eQQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774277349; c=relaxed/simple; bh=MbzkQ63xAlixK+E1ZsgcmGYb2DBxqCEnIDSnzCReO/k=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=LnRBhcjbstL/WKZe9ZgkOvjXThL0IjE43XJ6YyEF6sf/wliY9SoFbgdBst4nkiGN05ORljulvdBub3NEJ9BWN4tPTax8AQmSoTOdc7SovEKkl5r4dxO7BEOmXkwQs0bDO4gWQMReGxuyUplsbaQ43ovrlXM9+f7H5T5YkKtxrlw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=XLME0Fpf; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="XLME0Fpf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D9538C4CEF7; Mon, 23 Mar 2026 14:49:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774277349; bh=MbzkQ63xAlixK+E1ZsgcmGYb2DBxqCEnIDSnzCReO/k=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=XLME0Fpf1Lm7W+t9ol5Snw5wh+yTyrd0PHJ+Qj9ux0wgOMSxboO7wAQCX1JozQaJV HglzILSR52IuyTUzOe2NpJjDxDaw6wiK9oHtN5I9Z0MGyAsVoF2IWQScNQKznxIMJe A2oxRkJRhTdhkLjS0GrQCpvVpCAeqwyzDfIASwBw= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+827ae2bfb3a3529333e9@syzkaller.appspotmail.com, Eric Dumazet , Jiayuan Chen , Jakub Kicinski , Sasha Levin Subject: [PATCH 6.12 406/460] net/smc: fix NULL dereference and UAF in smc_tcp_syn_recv_sock() Date: Mon, 23 Mar 2026 14:46:42 +0100 Message-ID: <20260323134536.535123012@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134526.647552166@linuxfoundation.org> References: <20260323134526.647552166@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jiayuan Chen [ Upstream commit 6d5e4538364b9ceb1ac2941a4deb86650afb3538 ] 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 Fixes: 8270d9c21041 ("net/smc: Limit backlog connections") Reported-by: syzbot+827ae2bfb3a3529333e9@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/67eaf9b8.050a0220.3c3d88.004a.GAE@google.com/T/ Suggested-by: Eric Dumazet Reviewed-by: Eric Dumazet Signed-off-by: Jiayuan Chen Link: https://patch.msgid.link/20260312092909.48325-1-jiayuan.chen@linux.dev Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- net/smc/af_smc.c | 23 +++++++++++++++++------ net/smc/smc.h | 5 +++++ net/smc/smc_close.c | 2 +- 3 files changed, 23 insertions(+), 7 deletions(-) diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c index 02e08ac1da3aa..23bb360ebd07b 100644 --- a/net/smc/af_smc.c +++ b/net/smc/af_smc.c @@ -130,7 +130,14 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk, struct smc_sock *smc; struct sock *child; - smc = smc_clcsock_user_data(sk); + rcu_read_lock(); + smc = smc_clcsock_user_data_rcu(sk); + if (!smc || !refcount_inc_not_zero(&smc->sk.sk_refcnt)) { + rcu_read_unlock(); + smc = NULL; + goto drop; + } + rcu_read_unlock(); if (READ_ONCE(sk->sk_ack_backlog) + atomic_read(&smc->queued_smc_hs) > sk->sk_max_ack_backlog) @@ -152,11 +159,14 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk, if (inet_csk(child)->icsk_af_ops == inet_csk(sk)->icsk_af_ops) inet_csk(child)->icsk_af_ops = smc->ori_af_ops; } + sock_put(&smc->sk); return child; drop: dst_release(dst); tcp_listendrop(sk); + if (smc) + sock_put(&smc->sk); return NULL; } @@ -253,7 +263,7 @@ static void smc_fback_restore_callbacks(struct smc_sock *smc) struct sock *clcsk = smc->clcsock->sk; write_lock_bh(&clcsk->sk_callback_lock); - clcsk->sk_user_data = NULL; + rcu_assign_sk_user_data(clcsk, NULL); smc_clcsock_restore_cb(&clcsk->sk_state_change, &smc->clcsk_state_change); smc_clcsock_restore_cb(&clcsk->sk_data_ready, &smc->clcsk_data_ready); @@ -901,7 +911,7 @@ static void smc_fback_replace_callbacks(struct smc_sock *smc) struct sock *clcsk = smc->clcsock->sk; write_lock_bh(&clcsk->sk_callback_lock); - clcsk->sk_user_data = (void *)((uintptr_t)smc | SK_USER_DATA_NOCOPY); + __rcu_assign_sk_user_data_with_flags(clcsk, smc, SK_USER_DATA_NOCOPY); smc_clcsock_replace_cb(&clcsk->sk_state_change, smc_fback_state_change, &smc->clcsk_state_change); @@ -2663,8 +2673,8 @@ int smc_listen(struct socket *sock, int backlog) * smc-specific sk_data_ready function */ write_lock_bh(&smc->clcsock->sk->sk_callback_lock); - smc->clcsock->sk->sk_user_data = - (void *)((uintptr_t)smc | SK_USER_DATA_NOCOPY); + __rcu_assign_sk_user_data_with_flags(smc->clcsock->sk, smc, + SK_USER_DATA_NOCOPY); smc_clcsock_replace_cb(&smc->clcsock->sk->sk_data_ready, smc_clcsock_data_ready, &smc->clcsk_data_ready); write_unlock_bh(&smc->clcsock->sk->sk_callback_lock); @@ -2685,10 +2695,11 @@ int smc_listen(struct socket *sock, int backlog) write_lock_bh(&smc->clcsock->sk->sk_callback_lock); smc_clcsock_restore_cb(&smc->clcsock->sk->sk_data_ready, &smc->clcsk_data_ready); - smc->clcsock->sk->sk_user_data = NULL; + rcu_assign_sk_user_data(smc->clcsock->sk, NULL); write_unlock_bh(&smc->clcsock->sk->sk_callback_lock); goto out; } + sock_set_flag(sk, SOCK_RCU_FREE); sk->sk_max_ack_backlog = backlog; sk->sk_ack_backlog = 0; sk->sk_state = SMC_LISTEN; diff --git a/net/smc/smc.h b/net/smc/smc.h index 7579f9622e010..f9d364a2167a7 100644 --- a/net/smc/smc.h +++ b/net/smc/smc.h @@ -346,6 +346,11 @@ static inline struct smc_sock *smc_clcsock_user_data(const struct sock *clcsk) ((uintptr_t)clcsk->sk_user_data & ~SK_USER_DATA_NOCOPY); } +static inline struct smc_sock *smc_clcsock_user_data_rcu(const struct sock *clcsk) +{ + return (struct smc_sock *)rcu_dereference_sk_user_data(clcsk); +} + /* save target_cb in saved_cb, and replace target_cb with new_cb */ static inline void smc_clcsock_replace_cb(void (**target_cb)(struct sock *), void (*new_cb)(struct sock *), diff --git a/net/smc/smc_close.c b/net/smc/smc_close.c index 10219f55aad14..bb0313ef5f7c1 100644 --- a/net/smc/smc_close.c +++ b/net/smc/smc_close.c @@ -218,7 +218,7 @@ int smc_close_active(struct smc_sock *smc) write_lock_bh(&smc->clcsock->sk->sk_callback_lock); smc_clcsock_restore_cb(&smc->clcsock->sk->sk_data_ready, &smc->clcsk_data_ready); - smc->clcsock->sk->sk_user_data = NULL; + rcu_assign_sk_user_data(smc->clcsock->sk, NULL); write_unlock_bh(&smc->clcsock->sk->sk_callback_lock); rc = kernel_sock_shutdown(smc->clcsock, SHUT_RDWR); } -- 2.51.0