ax25: rcu protect dev->ax25_ptr

syzbot found a lockdep issue [1].

We should remove ax25 RTNL dependency in ax25_setsockopt()

This should also fix a variety of possible UAF in ax25.

[1]

WARNING: possible circular locking dependency detected
6.13.0-rc3-syzkaller-00762-g9268abe611b0 #0 Not tainted
------------------------------------------------------
syz.5.1818/12806 is trying to acquire lock:
 ffffffff8fcb3988 (rtnl_mutex){+.+.}-{4:4}, at: ax25_setsockopt+0xa55/0xe90 net/ax25/af_ax25.c:680

but task is already holding lock:
 ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1618 [inline]
 ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: ax25_setsockopt+0x209/0xe90 net/ax25/af_ax25.c:574

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (sk_lock-AF_AX25){+.+.}-{0:0}:
        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
        lock_sock_nested+0x48/0x100 net/core/sock.c:3642
        lock_sock include/net/sock.h:1618 [inline]
        ax25_kill_by_device net/ax25/af_ax25.c:101 [inline]
        ax25_device_event+0x24d/0x580 net/ax25/af_ax25.c:146
        notifier_call_chain+0x1a5/0x3f0 kernel/notifier.c:85
       __dev_notify_flags+0x207/0x400
        dev_change_flags+0xf0/0x1a0 net/core/dev.c:9026
        dev_ifsioc+0x7c8/0xe70 net/core/dev_ioctl.c:563
        dev_ioctl+0x719/0x1340 net/core/dev_ioctl.c:820
        sock_do_ioctl+0x240/0x460 net/socket.c:1234
        sock_ioctl+0x626/0x8e0 net/socket.c:1339
        vfs_ioctl fs/ioctl.c:51 [inline]
        __do_sys_ioctl fs/ioctl.c:906 [inline]
        __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
        do_syscall_x64 arch/x86/entry/common.c:52 [inline]
        do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (rtnl_mutex){+.+.}-{4:4}:
        check_prev_add kernel/locking/lockdep.c:3161 [inline]
        check_prevs_add kernel/locking/lockdep.c:3280 [inline]
        validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
        __lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
        __mutex_lock_common kernel/locking/mutex.c:585 [inline]
        __mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
        ax25_setsockopt+0xa55/0xe90 net/ax25/af_ax25.c:680
        do_sock_setsockopt+0x3af/0x720 net/socket.c:2324
        __sys_setsockopt net/socket.c:2349 [inline]
        __do_sys_setsockopt net/socket.c:2355 [inline]
        __se_sys_setsockopt net/socket.c:2352 [inline]
        __x64_sys_setsockopt+0x1ee/0x280 net/socket.c:2352
        do_syscall_x64 arch/x86/entry/common.c:52 [inline]
        do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(sk_lock-AF_AX25);
                               lock(rtnl_mutex);
                               lock(sk_lock-AF_AX25);
  lock(rtnl_mutex);

 *** DEADLOCK ***

1 lock held by syz.5.1818/12806:
  #0: ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1618 [inline]
  #0: ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: ax25_setsockopt+0x209/0xe90 net/ax25/af_ax25.c:574

stack backtrace:
CPU: 1 UID: 0 PID: 12806 Comm: syz.5.1818 Not tainted 6.13.0-rc3-syzkaller-00762-g9268abe611b0 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
 <TASK>
  __dump_stack lib/dump_stack.c:94 [inline]
  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
  print_circular_bug+0x13a/0x1b0 kernel/locking/lockdep.c:2074
  check_noncircular+0x36a/0x4a0 kernel/locking/lockdep.c:2206
  check_prev_add kernel/locking/lockdep.c:3161 [inline]
  check_prevs_add kernel/locking/lockdep.c:3280 [inline]
  validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
  __lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
  lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
  __mutex_lock_common kernel/locking/mutex.c:585 [inline]
  __mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
  ax25_setsockopt+0xa55/0xe90 net/ax25/af_ax25.c:680
  do_sock_setsockopt+0x3af/0x720 net/socket.c:2324
  __sys_setsockopt net/socket.c:2349 [inline]
  __do_sys_setsockopt net/socket.c:2355 [inline]
  __se_sys_setsockopt net/socket.c:2352 [inline]
  __x64_sys_setsockopt+0x1ee/0x280 net/socket.c:2352
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7b62385d29

Fixes: c433570 ("ax25: fix a use-after-free in ax25_fillin_cb()")
Reported-by: syzbot <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Eric Dumazet

include/linux/netdevice.h

@@ -2261,7 +2261,7 @@ struct net_device {
 	void 			*atalk_ptr;
 #endif
 #if IS_ENABLED(CONFIG_AX25)
-	void			*ax25_ptr;
+	struct ax25_dev	__rcu	*ax25_ptr;
 #endif
 #if IS_ENABLED(CONFIG_CFG80211)
 	struct wireless_dev	*ieee80211_ptr;

include/net/ax25.h

@@ -231,6 +231,7 @@ typedef struct ax25_dev {
 #endif
 	refcount_t		refcount;
 	bool device_up;
+	struct rcu_head		rcu;
 } ax25_dev;
 
 typedef struct ax25_cb {
@@ -290,9 +291,8 @@ static inline void ax25_dev_hold(ax25_dev *ax25_dev)
 
 static inline void ax25_dev_put(ax25_dev *ax25_dev)
 {
-	if (refcount_dec_and_test(&ax25_dev->refcount)) {
-		kfree(ax25_dev);
-	}
+	if (refcount_dec_and_test(&ax25_dev->refcount))
+		kfree_rcu(ax25_dev, rcu);
 }
 static inline __be16 ax25_type_trans(struct sk_buff *skb, struct net_device *dev)
 {
@@ -335,9 +335,9 @@ void ax25_digi_invert(const ax25_digi *, ax25_digi *);
 extern spinlock_t ax25_dev_lock;
 
 #if IS_ENABLED(CONFIG_AX25)
-static inline ax25_dev *ax25_dev_ax25dev(struct net_device *dev)
+static inline ax25_dev *ax25_dev_ax25dev(const struct net_device *dev)
 {
-	return dev->ax25_ptr;
+	return rcu_dereference_rtnl(dev->ax25_ptr);
 }
 #endif
 

net/ax25/af_ax25.c

@@ -467,7 +467,7 @@ static int ax25_ctl_ioctl(const unsigned int cmd, void __user *arg)
 	goto out_put;
 }
 
-static void ax25_fillin_cb_from_dev(ax25_cb *ax25, ax25_dev *ax25_dev)
+static void ax25_fillin_cb_from_dev(ax25_cb *ax25, const ax25_dev *ax25_dev)
 {
 	ax25->rtt     = msecs_to_jiffies(ax25_dev->values[AX25_VALUES_T1]) / 2;
 	ax25->t1      = msecs_to_jiffies(ax25_dev->values[AX25_VALUES_T1]);
@@ -677,22 +677,22 @@ static int ax25_setsockopt(struct socket *sock, int level, int optname,
 			break;
 		}
 
-		rtnl_lock();
-		dev = __dev_get_by_name(&init_net, devname);
+		rcu_read_lock();
+		dev = dev_get_by_name_rcu(&init_net, devname);
 		if (!dev) {
-			rtnl_unlock();
+			rcu_read_unlock();
 			res = -ENODEV;
 			break;
 		}
 
 		ax25->ax25_dev = ax25_dev_ax25dev(dev);
 		if (!ax25->ax25_dev) {
-			rtnl_unlock();
+			rcu_read_unlock();
 			res = -ENODEV;
 			break;
 		}
 		ax25_fillin_cb(ax25, ax25->ax25_dev);
-		rtnl_unlock();
+		rcu_read_unlock();
 		break;
 
 	default:

net/ax25/ax25_dev.c

@@ -90,7 +90,7 @@ void ax25_dev_device_up(struct net_device *dev)
 
 	spin_lock_bh(&ax25_dev_lock);
 	list_add(&ax25_dev->list, &ax25_dev_list);
-	dev->ax25_ptr     = ax25_dev;
+	rcu_assign_pointer(dev->ax25_ptr, ax25_dev);
 	spin_unlock_bh(&ax25_dev_lock);
 
 	ax25_register_dev_sysctl(ax25_dev);
@@ -125,7 +125,7 @@ void ax25_dev_device_down(struct net_device *dev)
 		}
 	}
 
-	dev->ax25_ptr = NULL;
+	RCU_INIT_POINTER(dev->ax25_ptr, NULL);
 	spin_unlock_bh(&ax25_dev_lock);
 	netdev_put(dev, &ax25_dev->dev_tracker);
 	ax25_dev_put(ax25_dev);

net/ax25/ax25_ip.c

@@ -122,6 +122,7 @@ netdev_tx_t ax25_ip_xmit(struct sk_buff *skb)
 	if (dev == NULL)
 		dev = skb->dev;
 
+	rcu_read_lock();
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL) {
 		kfree_skb(skb);
 		goto put;
@@ -202,7 +203,7 @@ netdev_tx_t ax25_ip_xmit(struct sk_buff *skb)
 	ax25_queue_xmit(skb, dev);
 
 put:
-
+	rcu_read_unlock();
 	ax25_route_lock_unuse();
 	return NETDEV_TX_OK;
 }

net/ax25/ax25_out.c

@@ -39,10 +39,14 @@ ax25_cb *ax25_send_frame(struct sk_buff *skb, int paclen, const ax25_address *sr
 	 * specified.
 	 */
 	if (paclen == 0) {
-		if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
+		rcu_read_lock();
+		ax25_dev = ax25_dev_ax25dev(dev);
+		if (!ax25_dev) {
+			rcu_read_unlock();
 			return NULL;
-
+		}
 		paclen = ax25_dev->values[AX25_VALUES_PACLEN];
+		rcu_read_unlock();
 	}
 
 	/*
@@ -53,13 +57,19 @@ ax25_cb *ax25_send_frame(struct sk_buff *skb, int paclen, const ax25_address *sr
 		return ax25;		/* It already existed */
 	}
 
-	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
+	rcu_read_lock();
+	ax25_dev = ax25_dev_ax25dev(dev);
+	if (!ax25_dev) {
+		rcu_read_unlock();
 		return NULL;
+	}
 
-	if ((ax25 = ax25_create_cb()) == NULL)
+	if ((ax25 = ax25_create_cb()) == NULL) {
+		rcu_read_unlock();
 		return NULL;
-
+	}
 	ax25_fillin_cb(ax25, ax25_dev);
+	rcu_read_unlock();
 
 	ax25->source_addr = *src;
 	ax25->dest_addr   = *dest;
@@ -358,7 +368,9 @@ void ax25_queue_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	unsigned char *ptr;
 
+	rcu_read_lock();
 	skb->protocol = ax25_type_trans(skb, ax25_fwd_dev(dev));
+	rcu_read_unlock();
 
 	ptr  = skb_push(skb, 1);
 	*ptr = 0x00;			/* KISS */

net/ax25/ax25_route.c

@@ -406,6 +406,7 @@ int ax25_rt_autobind(ax25_cb *ax25, ax25_address *addr)
 		ax25_route_lock_unuse();
 		return -EHOSTUNREACH;
 	}
+	rcu_read_lock();
 	if ((ax25->ax25_dev = ax25_dev_ax25dev(ax25_rt->dev)) == NULL) {
 		err = -EHOSTUNREACH;
 		goto put;
@@ -442,6 +443,7 @@ int ax25_rt_autobind(ax25_cb *ax25, ax25_address *addr)
 	}
 
 put:
+	rcu_read_unlock();
 	ax25_route_lock_unuse();
 	return err;
 }