netlink: don't call ->netlink_bind with table lock held

When I added support to allow generic netlink multicast groups to be
restricted to subscribers with CAP_NET_ADMIN I was unaware that a
genl_bind implementation already existed in the past.

It was reverted due to ABBA deadlock:

1. ->netlink_bind gets called with the table lock held.
2. genetlink bind callback is invoked, it grabs the genl lock.

But when a new genl subsystem is (un)registered, these two locks are
taken in reverse order.

One solution would be to revert again and add a comment in genl
referring 1e82a62fec613, "genetlink: remove genl_bind").

This would need a second change in mptcp to not expose the raw token
value anymore, e.g.  by hashing the token with a secret key so userspace
can still associate subflow events with the correct mptcp connection.

However, Paolo Abeni reminded me to double-check why the netlink table is
locked in the first place.

I can't find one.  netlink_bind() is already called without this lock
when userspace joins a group via NETLINK_ADD_MEMBERSHIP setsockopt.
Same holds for the netlink_unbind operation.

Digging through the history, commit f773608026ee1
("netlink: access nlk groups safely in netlink bind and getname")
expanded the lock scope.

commit 3a20773beeeeade ("net: netlink: cap max groups which will be considered in netlink_bind()")
... removed the nlk->ngroups access that the lock scope
extension was all about.

Reduce the lock scope again and always call ->netlink_bind without
the table lock.

The Fixes tag should be vs. the patch mentioned in the link below,
but that one got squash-merged into the patch that came earlier in the
series.

Fixes: 4d54cc32112d8d ("mptcp: avoid lock_fast usage in accept path")
Link: https://lore.kernel.org/mptcp/20210213000001.379332-8-mathew.j.martineau@linux.intel.com/T/#u
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Sean Tranchetti <stranche@codeaurora.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index dd488938447f9735daf1fb727c339a9874bab38b..3a62f97acf39d11a18547308a2f375eb5f37abaa 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1019,7 +1019,6 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
                        return -EINVAL;
        }
 
                        return -EINVAL;
        }
 

-       netlink_lock_table();

        if (nlk->netlink_bind && groups) {
                int group;
 
        if (nlk->netlink_bind && groups) {
                int group;
 


@@ -1031,13 +1030,14 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
                        if (!err)
                                continue;
                        netlink_undo_bind(group, groups, sk);
                        if (!err)
                                continue;
                        netlink_undo_bind(group, groups, sk);

-                       goto unlock;
+                       return err;

                }
        }
 
        /* No need for barriers here as we return to user-space without
         * using any of the bound attributes.
         */
                }
        }
 
        /* No need for barriers here as we return to user-space without
         * using any of the bound attributes.
         */

+       netlink_lock_table();

        if (!bound) {
                err = nladdr->nl_pid ?
                        netlink_insert(sk, nladdr->nl_pid) :
        if (!bound) {
                err = nladdr->nl_pid ?
                        netlink_insert(sk, nladdr->nl_pid) :