[PATCH net v2] genetlink: remove genl_bind

[PATCH net v2] genetlink: remove genl_bind

[Sean Tranchetti]

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 <xiyou.wangcong@gmail.com>
Acked-by: Johannes Berg <johannes.berg@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
---
 include/net/genetlink.h |  8 --------
 net/netlink/genetlink.c | 49 -------------------------------------------------
 2 files changed, 57 deletions(-)

diff --git a/include/net/genetlink.h b/include/net/genetlink.h
index 7495066..4cf703d 100644
--- a/include/net/genetlink.h
+++ b/include/net/genetlink.h
@@ -35,12 +35,6 @@ struct genl_multicast_group {
  *	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.
  * @attrbuf: buffer to store parsed attributes (private)
  * @mcgrps: multicast groups used by this family
  * @n_mcgrps: number of multicast groups
@@ -64,8 +58,6 @@ 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);
 	struct nlattr **	attrbuf;	/* private */
 	const struct genl_ops *	ops;
 	const struct genl_multicast_group *mcgrps;
diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index 55ee680..dfaaefb 100644
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -1166,60 +1166,11 @@ static int ctrl_dumppolicy(struct sk_buff *skb, struct netlink_callback *cb)
 	.netnsok = true,
 };
 
-static int genl_bind(struct net *net, int group)
-{
-	struct genl_family *f;
-	int err = -ENOENT;
-	unsigned int id;
-
-	down_read(&cb_lock);
-
-	idr_for_each_entry(&genl_fam_idr, f, id) {
-		if (group >= f->mcgrp_offset &&
-		    group < f->mcgrp_offset + f->n_mcgrps) {
-			int fam_grp = group - f->mcgrp_offset;
-
-			if (!f->netnsok && net != &init_net)
-				err = -ENOENT;
-			else if (f->mcast_bind)
-				err = f->mcast_bind(net, fam_grp);
-			else
-				err = 0;
-			break;
-		}
-	}
-	up_read(&cb_lock);
-
-	return err;
-}
-
-static void genl_unbind(struct net *net, int group)
-{
-	struct genl_family *f;
-	unsigned int id;
-
-	down_read(&cb_lock);
-
-	idr_for_each_entry(&genl_fam_idr, f, id) {
-		if (group >= f->mcgrp_offset &&
-		    group < f->mcgrp_offset + f->n_mcgrps) {
-			int fam_grp = group - f->mcgrp_offset;
-
-			if (f->mcast_unbind)
-				f->mcast_unbind(net, fam_grp);
-			break;
-		}
-	}
-	up_read(&cb_lock);
-}
-
 static int __net_init genl_pernet_init(struct net *net)
 {
 	struct netlink_kernel_cfg cfg = {
 		.input		= genl_rcv,
 		.flags		= NL_CFG_F_NONROOT_RECV,
-		.bind		= genl_bind,
-		.unbind		= genl_unbind,
 	};
 
 	/* we'll bump the group number right afterwards */
-- 
1.9.1

Re: [PATCH net v2] genetlink: remove genl_bind

[Cong Wang]

On Tue, Jun 30, 2020 at 10:50 AM Sean Tranchetti
<stranche@codeaurora.org> wrote:
>
> 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.

I think it is worth noting removing the -ENOENT is probably okay,
as mentioned in commit 023e2cfa36c31b0ad28c159a1bb0d61ff57334c8:

    Also do this in generic netlink, and there also make sure that any
    bind for multicast groups that only exist in init_net is rejected.
    This isn't really a problem if it is accepted since a client in a
    different namespace will never receive any notifications from such
    a group, but it can confuse the family if not rejected (it's also
    possible to silently (without telling the family) accept it, but it
    would also have to be ignored on unbind so families that take any
    kind of action on bind/unbind won't do unnecessary work for invalid
    clients like that.

Thanks.

Re: [PATCH net v2] genetlink: remove genl_bind

[David Miller]

From: Sean Tranchetti <stranche@codeaurora.org>
Date: Tue, 30 Jun 2020 11:50:17 -0600

> 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 <xiyou.wangcong@gmail.com>
> Acked-by: Johannes Berg <johannes.berg@intel.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>

Applied and queued up for -stable, thanks.