Currently, netconsole cleans up the netpoll structure before disabling
the target. This approach can lead to race conditions, as message
senders (write_ext_msg() and write_msg()) check if the target is
enabled before using netpoll. The sender can validate that the target is
enabled, but, the netpoll might be de-allocated already, causing
undesired behaviours.

This patch reverses the order of operations:
1. Disable the target
2. Clean up the netpoll structure

This change eliminates the potential race condition, ensuring that
no messages are sent through a partially cleaned-up netpoll structure.

Fixes: 2382b15bcc39 ("netconsole: take care of NETDEV_UNREGISTER event")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changelog:

v2:
 * Targeting "net" instead of "net-dev" (Jakub)

v1:
 * https://lore.kernel.org/all/20240709144403.544099-4-leitao@debian.org/

 drivers/net/netconsole.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index d7070dd4fe73..aa66c923790f 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -974,6 +974,7 @@ static int netconsole_netdev_event(struct notifier_block *this,
 				/* rtnl_lock already held
 				 * we might sleep in __netpoll_cleanup()
 				 */
+				nt->enabled = false;
 				spin_unlock_irqrestore(&target_list_lock, flags);
 
 				__netpoll_cleanup(&nt->np);
@@ -981,7 +982,6 @@ static int netconsole_netdev_event(struct notifier_block *this,
 				spin_lock_irqsave(&target_list_lock, flags);
 				netdev_put(nt->np.dev, &nt->np.dev_tracker);
 				nt->np.dev = NULL;
-				nt->enabled = false;
 				stopped = true;
 				netconsole_target_put(nt);
 				goto restart;
-- 
2.43.0

On Fri, Jul 12, 2024 at 7:34 AM Breno Leitao <leitao@debian.org> wrote:
>
> Currently, netconsole cleans up the netpoll structure before disabling
> the target. This approach can lead to race conditions, as message
> senders (write_ext_msg() and write_msg()) check if the target is
> enabled before using netpoll. The sender can validate that the target is
> enabled, but, the netpoll might be de-allocated already, causing
> undesired behaviours.
>
> This patch reverses the order of operations:
> 1. Disable the target
> 2. Clean up the netpoll structure
>
> This change eliminates the potential race condition, ensuring that
> no messages are sent through a partially cleaned-up netpoll structure.
>
> Fixes: 2382b15bcc39 ("netconsole: take care of NETDEV_UNREGISTER event")
> Cc: stable@vger.kernel.org
> Signed-off-by: Breno Leitao <leitao@debian.org>

Goof catch, thanks.

Reviewed-by: Eric Dumazet <edumazet@google.com>