[PATCH v3] ax25: Fix refcount imbalance on inbound connections

[lars@oddbit.com]

From: Lars Kellogg-Stedman <lars@oddbit.com>

When releasing a socket in ax25_release(), we call netdev_put() to
decrease the refcount on the associated ax.25 device. However, the
execution path for accepting an incoming connection never calls
netdev_hold(). This imbalance leads to refcount errors, and ultimately
to kernel crashes.

A typical call trace for the above situation looks like this:

    Call Trace:
    <TASK>
    ? show_regs+0x64/0x70
    ? __warn+0x83/0x120
    ? refcount_warn_saturate+0xb2/0x100
    ? report_bug+0x158/0x190
    ? prb_read_valid+0x20/0x30
    ? handle_bug+0x3e/0x70
    ? exc_invalid_op+0x1c/0x70
    ? asm_exc_invalid_op+0x1f/0x30
    ? refcount_warn_saturate+0xb2/0x100
    ? refcount_warn_saturate+0xb2/0x100
    ax25_release+0x2ad/0x360
    __sock_release+0x35/0xa0
    sock_close+0x19/0x20
    [...]

On reboot (or any attempt to remove the interface), the kernel gets
stuck in an infinite loop:

    unregister_netdevice: waiting for ax0 to become free. Usage count = 0

This patch corrects these issues by ensuring that we call netdev_hold()
and ax25_dev_hold() for new connections in ax25_accept().

Fixes: 7d8a3a477b
Signed-off-by: Lars Kellogg-Stedman <lars@oddbit.com>
---
v3:
- Address naveenm's comments regarding the ordering of variable
  declarations (https://lore.kernel.org/netdev/SJ2PR18MB5635B7ADC7339BEDB79B183DA2EA2@SJ2PR18MB5635.namprd18.prod.outlook.com/)

 net/ax25/af_ax25.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 8077cf2ee44..d6f9fae06a9 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -1378,8 +1378,10 @@ static int ax25_accept(struct socket *sock, struct socket *newsock,
 {
 	struct sk_buff *skb;
 	struct sock *newsk;
+	ax25_dev *ax25_dev;
 	DEFINE_WAIT(wait);
 	struct sock *sk;
+	ax25_cb *ax25;
 	int err = 0;
 
 	if (sock->state != SS_UNCONNECTED)
@@ -1434,6 +1436,10 @@ static int ax25_accept(struct socket *sock, struct socket *newsock,
 	kfree_skb(skb);
 	sk_acceptq_removed(sk);
 	newsock->state = SS_CONNECTED;
+	ax25 = sk_to_ax25(newsk);
+	ax25_dev = ax25->ax25_dev;
+	netdev_hold(ax25_dev->dev, &ax25->dev_tracker, GFP_ATOMIC);
+	ax25_dev_hold(ax25_dev);
 
 out:
 	release_sock(sk);
-- 
2.45.1