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: c433570458e4 ("ax25: fix a use-after-free in ax25_fillin_cb()")
Reported-by: syzbot <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/netdevice.h

···
 	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

···
 #endif
 	refcount_t		refcount;
 	bool device_up;
+	struct rcu_head		rcu;
 } ax25_dev;
 
 typedef struct ax25_cb {
···
 
 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)
 {
···
 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

···
 	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]);
···
 			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

···
 
 	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);
···
 		}
 	}
 
-	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

···
 	if (dev == NULL)
 		dev = skb->dev;
 
+	rcu_read_lock();
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL) {
 		kfree_skb(skb);
 		goto put;
···
 	ax25_queue_xmit(skb, dev);
 
 put:
-
+	rcu_read_unlock();
 	ax25_route_lock_unuse();
 	return NETDEV_TX_OK;
 }

net/ax25/ax25_out.c

···
 	 * 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();
 	}
 
 	/*
···
 		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;
···
 {
 	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

···
 		ax25_route_lock_unuse();
 		return -EHOSTUNREACH;
 	}
+	rcu_read_lock();
 	if ((ax25->ax25_dev = ax25_dev_ax25dev(ax25_rt->dev)) == NULL) {
 		err = -EHOSTUNREACH;
 		goto put;
···
 	}
 
 put:
+	rcu_read_unlock();
 	ax25_route_lock_unuse();
 	return err;
 }