[PATCH net-next 1/4] netconsole: finish enabling the target before releasing RTNL

[Gustavo Luiz Duarte <gustavold@gmail.com>]

netcons_netpoll_setup() publishes nt->np.dev inside __netpoll_setup(),
then releases the RTNL and waits for an RCU grace period before
returning. Only after that does its caller store STATE_ENABLED:

A NETDEV_UNREGISTER event landing in that window tears the target down
and we end up storing STATE_ENABLED with a NULL np->dev, leading to a
null-ptr-deref in netconsole_write():

  BUG: KASAN: null-ptr-deref in netconsole_write+0xb9/0x7f0
  Read of size 8 at addr 00000000000000a8 by task pr/netcon_ext0/135
   netconsole_write+0xb9/0x7f0
   nbcon_emit_next_record+0x501/0x550
   nbcon_emit_one+0x10e/0x170
   nbcon_kthread_func+0x2ff/0x3b0
   kthread+0x199/0x1e0

Store STATE_ENABLED before dropping the RTNL lock to avoid racing with
netconsole_netdev_event().

Fixes: 2382b15bcc39 ("netconsole: take care of NETDEV_UNREGISTER event")
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
 drivers/net/netconsole.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 267254f046de..7dde00eb8e98 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -552,13 +552,16 @@ static int netcons_netpoll_setup(struct netconsole_target *nt)
 	err = __netpoll_setup(np, ndev);
 	if (err)
 		goto put;
-	rtnl_unlock();
 
 	/* Make sure all NAPI polls which started before dev->npinfo
 	 * was visible have exited before we start calling NAPI poll.
 	 * NAPI skips locking if dev->npinfo is NULL.
+	 * Hold RTNL until enable is finished so we don't race with
+	 * netconsole_netdev_event()
 	 */
-	synchronize_rcu();
+	synchronize_net();
+	nt->state = STATE_ENABLED;
+	rtnl_unlock();
 
 	return 0;
 
@@ -589,7 +592,6 @@ static void resume_target(struct netconsole_target *nt)
 		return;
 	}
 
-	nt->state = STATE_ENABLED;
 	pr_info("network logging resumed on interface %s\n", nt->np.dev_name);
 }
 
@@ -1079,7 +1081,6 @@ static ssize_t enabled_store(struct config_item *item,
 			goto out_unlock;
 		}
 
-		nt->state = STATE_ENABLED;
 		pr_info("network logging started\n");
 	} else {	/* false */
 		/* We need to disable the netconsole before cleaning it up
@@ -2667,8 +2668,6 @@ static struct netconsole_target *alloc_param_target(char *target_config,
 			 * otherwise, keep the target in the list, but disabled.
 			 */
 			goto fail;
-	} else {
-		nt->state = STATE_ENABLED;
 	}
 	populate_configfs_item(nt, cmdline_count);
 

-- 
2.55.0