From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp153-168.sina.com.cn (smtp153-168.sina.com.cn [61.135.153.168]) (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 31F0334D4CF for ; Tue, 10 Mar 2026 03:27:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=61.135.153.168 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773113281; cv=none; b=sx+QLoq+jieJ1vZ/IWlrC7uJWitkdCiVRkfwi9igt0ifVZyOEi76m3DvivW0I2u/tWsCi16NaM+pbMirHUUvG+jPkwlyt4vL8iWiNlUFn1ALn2KOmSZD+4feFo8WYfkBKY2ABA+FThobdrdjWULhB6E5tZM/gi8v1ay4/GIHoAw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773113281; c=relaxed/simple; bh=3GTE2BLCNFDmKE+Ue5/OLTk2Qapxkp6/4dwpWTstFRE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Gl8kdw1Afk7S1SC6N/xI/AYL/wjajG1NTw8aI06SqygFirbsU1eUFiCb2jBtMw4K2Uzf/nEuXNcMj3lfqmGOWOkRSkTGc7t8ocWG2ILlPnLpvdiUOlMJ5nDcCZhP/7cs5hn3g1OpXJjq3N4+5mpr2DuJ2gs+F3FQun2gMi9yVb0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com; spf=pass smtp.mailfrom=sina.com; dkim=pass (1024-bit key) header.d=sina.com header.i=@sina.com header.b=WYExmZzD; arc=none smtp.client-ip=61.135.153.168 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sina.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=sina.com header.i=@sina.com header.b="WYExmZzD" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sina.com; s=201208; t=1773113274; bh=5/m5WGbRVXtJuhdQ34UpLwjS+/NlykDt8Y/BbaUqvCA=; h=From:Subject:Date:Message-ID; b=WYExmZzDqV/WAJP1sUCKIiXE2hsUFYjPPgWAjv1ckcmkeMQDn9+0/rwQnzHJFad1E K9etBfjDMwK2GFW4SxXEf80/w0l3OQeqBCbatjN2p/xFTTZuvBNq9LVO9cwcJ7/Uuz auaFIgTb/Qeohc1Yv6MXNBVjOUrmhmxJZrFkBpj0= X-SMAIL-HELO: localhost.localdomain Received: from unknown (HELO localhost.localdomain)([114.249.62.144]) by sina.com (10.54.253.33) with ESMTP id 69AF8D730000173F; Tue, 10 Mar 2026 11:18:14 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 8737146685004 X-SMAIL-UIID: 0382335DBF6D47A09D3C1E697C097572-20260310-111814-1 From: Hillf Danton To: Deepanshu Kartikey Cc: edumazet@google.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+f50072212ab792c86925@syzkaller.appspotmail.com Subject: Re: [PATCH v2] atm: lec: fix use-after-free in sock_def_readable() Date: Tue, 10 Mar 2026 11:18:04 +0800 Message-ID: <20260310031805.752-1-hdanton@sina.com> In-Reply-To: <20260309155908.508768-1-kartikey406@gmail.com> References: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Mon, 9 Mar 2026 21:29:08 +0530 Deepanshu Kartikey wrote: > A race condition exists between lec_atm_close() setting priv->lecd > to NULL and concurrent access to priv->lecd in send_to_lecd(), > lec_handle_bridge(), and lec_atm_send(). When the socket is freed > via RCU while another thread is still using it, a use-after-free > occurs in sock_def_readable() when accessing the socket's wait queue. > > The root cause is that lec_atm_close() clears priv->lecd without > any synchronization, while callers dereference priv->lecd without > any protection against concurrent teardown. > > Fix this by converting priv->lecd to an RCU-protected pointer: > - Mark priv->lecd as __rcu in lec.h > - Use rcu_assign_pointer() in lec_atm_close() and lecd_attach() > for safe pointer assignment > - Use rcu_access_pointer() for NULL checks that do not dereference > the pointer in lec_start_xmit(), lec_push(), send_to_lecd() and > lecd_attach() > - Use rcu_read_lock/rcu_dereference/rcu_read_unlock in send_to_lecd(), > lec_handle_bridge() and lec_atm_send() to safely access lecd > - Use rcu_assign_pointer() followed by synchronize_rcu() in > lec_atm_close() to ensure all readers have completed before > proceeding. This is safe since lec_atm_close() is called from > vcc_release() which holds lock_sock(), a sleeping lock. > - Remove the manual sk_receive_queue drain from lec_atm_close() > since vcc_destroy_socket() already drains it after lec_atm_close() > returns. > > v2: Switch from spinlock + sock_hold/put approach to RCU to properly > fix the race. The v1 spinlock approach had two issues pointed out > by Eric Dumazet: > 1. priv->lecd was still accessed directly after releasing the > lock instead of using a local copy. > 2. The spinlock did not prevent packets being queued after > lec_atm_close() drains sk_receive_queue since timer and > workqueue paths bypass netif_stop_queue(). > > Note: Syzbot patch testing was attempted but the test VM terminated > unexpectedly with "Connection to localhost closed by remote host", > likely due to a QEMU AHCI emulation issue unrelated to this fix. > Compile testing with "make W=1 net/atm/lec.o" passes cleanly. > > Reported-by: syzbot+f50072212ab792c86925@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=f50072212ab792c86925 > Link: https://lore.kernel.org/all/20260309093614.502094-1-kartikey406@gmail.com/T/ [v1] > Signed-off-by: Deepanshu Kartikey > --- > net/atm/lec.c | 70 +++++++++++++++++++++++++++++++++------------------ > net/atm/lec.h | 2 +- > 2 files changed, 46 insertions(+), 26 deletions(-) > > diff --git a/net/atm/lec.c b/net/atm/lec.c > index fb93c6e1c329..32adf28663a0 100644 > --- a/net/atm/lec.c > +++ b/net/atm/lec.c > @@ -154,10 +154,19 @@ static void lec_handle_bridge(struct sk_buff *skb, struct net_device *dev) > /* 0x01 is topology change */ > > priv = netdev_priv(dev); > - atm_force_charge(priv->lecd, skb2->truesize); > - sk = sk_atm(priv->lecd); > - skb_queue_tail(&sk->sk_receive_queue, skb2); > - sk->sk_data_ready(sk); > + struct atm_vcc *vcc; > + > + rcu_read_lock(); > + vcc = rcu_dereference(priv->lecd); > + if (vcc) { > + atm_force_charge(vcc, skb2->truesize); > + sk = sk_atm(vcc); > + skb_queue_tail(&sk->sk_receive_queue, skb2); > + sk->sk_data_ready(sk); > + } else { > + dev_kfree_skb(skb2); > + } > + rcu_read_unlock(); > } > } > #endif /* IS_ENABLED(CONFIG_BRIDGE) */ > @@ -216,7 +225,7 @@ static netdev_tx_t lec_start_xmit(struct sk_buff *skb, > int is_rdesc; > > pr_debug("called\n"); > - if (!priv->lecd) { > + if (!rcu_access_pointer(priv->lecd)) { > pr_info("%s:No lecd attached\n", dev->name); > dev->stats.tx_errors++; > netif_stop_queue(dev); > @@ -449,10 +459,19 @@ static int lec_atm_send(struct atm_vcc *vcc, struct sk_buff *skb) > break; > skb2->len = sizeof(struct atmlec_msg); > skb_copy_to_linear_data(skb2, mesg, sizeof(*mesg)); > - atm_force_charge(priv->lecd, skb2->truesize); > - sk = sk_atm(priv->lecd); > - skb_queue_tail(&sk->sk_receive_queue, skb2); > - sk->sk_data_ready(sk); > + struct atm_vcc *vcc; > + > + rcu_read_lock(); > + vcc = rcu_dereference(priv->lecd); > + if (vcc) { > + atm_force_charge(vcc, skb2->truesize); > + sk = sk_atm(vcc); > + skb_queue_tail(&sk->sk_receive_queue, skb2); > + sk->sk_data_ready(sk); > + } else { > + dev_kfree_skb(skb2); > + } > + rcu_read_unlock(); > } > } > #endif /* IS_ENABLED(CONFIG_BRIDGE) */ > @@ -468,23 +486,16 @@ static int lec_atm_send(struct atm_vcc *vcc, struct sk_buff *skb) > > static void lec_atm_close(struct atm_vcc *vcc) > { > - struct sk_buff *skb; > struct net_device *dev = (struct net_device *)vcc->proto_data; > struct lec_priv *priv = netdev_priv(dev); > > - priv->lecd = NULL; > + rcu_assign_pointer(priv->lecd, NULL); > + synchronize_rcu(); > /* Do something needful? */ > At this point priv->lecd is no longer used, so why not make lecd valid throughout the lifespan of priv and free it after stopping dev queue, instead of the tedious rcu trick? > netif_stop_queue(dev); > lec_arp_destroy(priv); > > - if (skb_peek(&sk_atm(vcc)->sk_receive_queue)) > - pr_info("%s closing with messages pending\n", dev->name); > - while ((skb = skb_dequeue(&sk_atm(vcc)->sk_receive_queue))) { > - atm_return(vcc, skb->truesize); > - dev_kfree_skb(skb); > - } > - > pr_info("%s: Shut down!\n", dev->name); > module_put(THIS_MODULE); > } > @@ -510,12 +521,14 @@ send_to_lecd(struct lec_priv *priv, atmlec_msg_type type, > const unsigned char *mac_addr, const unsigned char *atm_addr, > struct sk_buff *data) > { > + struct atm_vcc *vcc; > struct sock *sk; > struct sk_buff *skb; > struct atmlec_msg *mesg; > > - if (!priv || !priv->lecd) > + if (!priv || !rcu_access_pointer(priv->lecd)) > return -1; > + > skb = alloc_skb(sizeof(struct atmlec_msg), GFP_ATOMIC); > if (!skb) > return -1; > @@ -532,18 +545,27 @@ send_to_lecd(struct lec_priv *priv, atmlec_msg_type type, > if (atm_addr) > memcpy(&mesg->content.normal.atm_addr, atm_addr, ATM_ESA_LEN); > > - atm_force_charge(priv->lecd, skb->truesize); > - sk = sk_atm(priv->lecd); > + rcu_read_lock(); > + vcc = rcu_dereference(priv->lecd); > + if (!vcc) { > + rcu_read_unlock(); > + kfree_skb(skb); > + return -1; > + } > + > + atm_force_charge(vcc, skb->truesize); > + sk = sk_atm(vcc); > skb_queue_tail(&sk->sk_receive_queue, skb); > sk->sk_data_ready(sk); > > if (data != NULL) { > pr_debug("about to send %d bytes of data\n", data->len); > - atm_force_charge(priv->lecd, data->truesize); > + atm_force_charge(vcc, data->truesize); > skb_queue_tail(&sk->sk_receive_queue, data); > sk->sk_data_ready(sk); > } > > + rcu_read_unlock(); > return 0; > } > > @@ -618,7 +640,7 @@ static void lec_push(struct atm_vcc *vcc, struct sk_buff *skb) > > atm_return(vcc, skb->truesize); > if (*(__be16 *) skb->data == htons(priv->lecid) || > - !priv->lecd || !(dev->flags & IFF_UP)) { > + !rcu_access_pointer(priv->lecd) || !(dev->flags & IFF_UP)) { > /* > * Probably looping back, or if lecd is missing, > * lecd has gone down > @@ -753,12 +775,12 @@ static int lecd_attach(struct atm_vcc *vcc, int arg) > priv = netdev_priv(dev_lec[i]); > } else { > priv = netdev_priv(dev_lec[i]); > - if (priv->lecd) > + if (rcu_access_pointer(priv->lecd)) > return -EADDRINUSE; > } > lec_arp_init(priv); > priv->itfnum = i; /* LANE2 addition */ > - priv->lecd = vcc; > + rcu_assign_pointer(priv->lecd, vcc); > vcc->dev = &lecatm_dev; > vcc_insert_socket(sk_atm(vcc)); > > diff --git a/net/atm/lec.h b/net/atm/lec.h > index be0e2667bd8c..ec85709bf818 100644 > --- a/net/atm/lec.h > +++ b/net/atm/lec.h > @@ -91,7 +91,7 @@ struct lec_priv { > */ > spinlock_t lec_arp_lock; > struct atm_vcc *mcast_vcc; /* Default Multicast Send VCC */ > - struct atm_vcc *lecd; > + struct atm_vcc __rcu *lecd; > struct delayed_work lec_arp_work; /* C10 */ > unsigned int maximum_unknown_frame_count; > /* > -- > 2.43.0