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/[email protected]/T/#u
Cc: Cong Wang <[email protected]>
Cc: Xin Long <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: Sean Tranchetti <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Pablo Neira Ayuso <[email protected]>
Signed-off-by: Florian Westphal <[email protected]>
---
net/netlink/af_netlink.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index dd488938447f..3a62f97acf39 100644
--- 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;
}
- netlink_lock_table();
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);
- goto unlock;
+ return err;
}
}
/* 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) :
--
2.26.3
