[PATCH] net: netpoll: ensure skb_pool list is always initialized

[John Sperbeck <jsperbeck@google.com>]

When __netpoll_setup() is called directly, instead of through
netpoll_setup(), the np->skb_pool list head isn't initialized.
If skb_pool_flush() is later called, then we hit a NULL pointer
in skb_queue_purge_reason().  This can be seen with this repro,
when CONFIG_NETCONSOLE is enabled as a module:

    ip tuntap add mode tap tap0
    ip link add name br0 type bridge
    ip link set dev tap0 master br0
    modprobe netconsole netconsole=4444@10.0.0.1/br0,9353@10.0.0.2/
    rmmod netconsole

The backtrace is:

    BUG: kernel NULL pointer dereference, address: 0000000000000008
    #PF: supervisor write access in kernel mode
    #PF: error_code(0x0002) - not-present page
    ... ... ...
    Call Trace:
     <TASK>
     __netpoll_free+0xa5/0xf0
     br_netpoll_cleanup+0x43/0x50 [bridge]
     do_netpoll_cleanup+0x43/0xc0
     netconsole_netdev_event+0x1e3/0x300 [netconsole]
     unregister_netdevice_notifier+0xd9/0x150
     cleanup_module+0x45/0x920 [netconsole]
     __se_sys_delete_module+0x205/0x290
     do_syscall_64+0x70/0x150
     entry_SYSCALL_64_after_hwframe+0x76/0x7e

Move the skb_pool list initialization into __netpoll_setup().  Also,
have netpoll_setup() call this before allocating its initial pool of
packets.

Fixes: 6c59f16f1770 ("net: netpoll: flush skb pool during cleanup")
Signed-off-by: John Sperbeck <jsperbeck@google.com>
---
 net/core/netpoll.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 2e459b9d88eb..61662390414e 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -627,6 +627,8 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 	const struct net_device_ops *ops;
 	int err;
 
+	skb_queue_head_init(&np->skb_pool);
+
 	if (ndev->priv_flags & IFF_DISABLE_NETPOLL) {
 		np_err(np, "%s doesn't support polling, aborting\n",
 		       ndev->name);
@@ -681,8 +683,6 @@ int netpoll_setup(struct netpoll *np)
 	struct in_device *in_dev;
 	int err;
 
-	skb_queue_head_init(&np->skb_pool);
-
 	rtnl_lock();
 	if (np->dev_name[0]) {
 		struct net *net = current->nsproxy->net_ns;
@@ -782,17 +782,16 @@ int netpoll_setup(struct netpoll *np)
 		}
 	}
 
+	err = __netpoll_setup(np, ndev);
+	if (err)
+		goto put;
+
 	/* fill up the skb queue */
 	refill_skbs(np);
 
-	err = __netpoll_setup(np, ndev);
-	if (err)
-		goto flush;
 	rtnl_unlock();
 	return 0;
 
-flush:
-	skb_pool_flush(np);
 put:
 	DEBUG_NET_WARN_ON_ONCE(np->dev);
 	if (ip_overwritten)
-- 
2.47.1.613.gc27f4b7a9f-goog