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genetlink: remove genl_bind

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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: c380d9a7af ("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>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Sean Tranchetti 2020-06-30 11:50:17 -06:00 committed by David S. Miller
parent d3c54f7f18
commit 1e82a62fec
2 changed files with 0 additions and 57 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
include/net/genetlink.h
View File

@ -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;

49
net/netlink/genetlink.c
View File

@ -1144,60 +1144,11 @@ static struct genl_family genl_ctrl __ro_after_init = {
.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 */