From 1e82a62fec613844da9e558f3493540a5b7a7b67 Mon Sep 17 00:00:00 2001 From: Sean Tranchetti Date: Tue, 30 Jun 2020 11:50:17 -0600 Subject: genetlink: remove genl_bind A potential deadlock can occur during registering or unregistering a new generic netlink family between the main nl_table_lock and the cb_lock where each thread wants the lock held by the other, as demonstrated below. 1) Thread 1 is performing a netlink_bind() operation on a socket. As part of this call, it will call netlink_lock_table(), incrementing the nl_table_users count to 1. 2) Thread 2 is registering (or unregistering) a genl_family via the genl_(un)register_family() API. The cb_lock semaphore will be taken for writing. 3) Thread 1 will call genl_bind() as part of the bind operation to handle subscribing to GENL multicast groups at the request of the user. It will attempt to take the cb_lock semaphore for reading, but it will fail and be scheduled away, waiting for Thread 2 to finish the write. 4) Thread 2 will call netlink_table_grab() during the (un)registration call. However, as Thread 1 has incremented nl_table_users, it will not be able to proceed, and both threads will be stuck waiting for the other. genl_bind() is a noop, unless a genl_family implements the mcast_bind() function to handle setting up family-specific multicast operations. Since no one in-tree uses this functionality as Cong pointed out, simply removing the genl_bind() function will remove the possibility for deadlock, as there is no attempt by Thread 1 above to take the cb_lock semaphore. Fixes: c380d9a7afff ("genetlink: pass multicast bind/unbind to families") Suggested-by: Cong Wang Acked-by: Johannes Berg Reported-by: kernel test robot Signed-off-by: Sean Tranchetti Signed-off-by: David S. Miller --- include/net/genetlink.h | 8 -------- 1 file changed, 8 deletions(-) (limited to 'include') diff --git a/include/net/genetlink.h b/include/net/genetlink.h index ad71ed4f55ff..6e5f1e1aa822 100644 --- a/include/net/genetlink.h +++ b/include/net/genetlink.h @@ -35,12 +35,6 @@ struct genl_info; * do additional, common, filtering and return an error * @post_doit: called after an operation's doit callback, it may * undo operations done by pre_doit, for example release locks - * @mcast_bind: a socket bound to the given multicast group (which - * is given as the offset into the groups array) - * @mcast_unbind: a socket was unbound from the given multicast group. - * Note that unbind() will not be called symmetrically if the - * generic netlink family is removed while there are still open - * sockets. * @mcgrps: multicast groups used by this family * @n_mcgrps: number of multicast groups * @mcgrp_offset: starting number of multicast group IDs in this family @@ -63,8 +57,6 @@ struct genl_family { void (*post_doit)(const struct genl_ops *ops, struct sk_buff *skb, struct genl_info *info); - int (*mcast_bind)(struct net *net, int group); - void (*mcast_unbind)(struct net *net, int group); const struct genl_ops * ops; const struct genl_multicast_group *mcgrps; unsigned int n_ops; -- cgit v1.2.3-59-g8ed1b