[PATCH 5.10 2/4] genetlink: add CAP_NET_ADMIN test for multicast bind

[Ido Schimmel <idosch@nvidia.com>]

This is a partial backport of upstream commit 4d54cc32112d ("mptcp:
avoid lock_fast usage in accept path"). It is only a partial backport
because the patch in the link below was erroneously squash-merged into
upstream commit 4d54cc32112d ("mptcp: avoid lock_fast usage in accept
path"). Below is the original patch description from Florian Westphal:

"
genetlink sets NL_CFG_F_NONROOT_RECV for its netlink socket so anyone can
subscribe to multicast messages.

rtnetlink doesn't allow this unconditionally,  rtnetlink_bind() restricts
bind requests to CAP_NET_ADMIN for a few groups.

This allows to set GENL_UNS_ADMIN_PERM flag on genl mcast groups to
mandate CAP_NET_ADMIN.

This will be used by the upcoming mptcp netlink event facility which
exposes the token (mptcp connection identifier) to userspace.
"

Link: https://lore.kernel.org/mptcp/20210213000001.379332-8-mathew.j.martineau@linux.intel.com/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
---
 include/net/genetlink.h |  1 +
 net/netlink/genetlink.c | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/include/net/genetlink.h b/include/net/genetlink.h
index e55ec1597ce7..7cb3fa8310ed 100644
--- a/include/net/genetlink.h
+++ b/include/net/genetlink.h
@@ -14,6 +14,7 @@
  */
 struct genl_multicast_group {
 	char			name[GENL_NAMSIZ];
+	u8			flags;
 };
 
 struct genl_ops;
diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index 9fd7ba01b9f8..4dce39013d75 100644
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -1364,11 +1364,43 @@ static struct genl_family genl_ctrl __ro_after_init = {
 	.netnsok = true,
 };
 
+static int genl_bind(struct net *net, int group)
+{
+	const struct genl_family *family;
+	unsigned int id;
+	int ret = 0;
+
+	genl_lock_all();
+
+	idr_for_each_entry(&genl_fam_idr, family, id) {
+		const struct genl_multicast_group *grp;
+		int i;
+
+		if (family->n_mcgrps == 0)
+			continue;
+
+		i = group - family->mcgrp_offset;
+		if (i < 0 || i >= family->n_mcgrps)
+			continue;
+
+		grp = &family->mcgrps[i];
+		if ((grp->flags & GENL_UNS_ADMIN_PERM) &&
+		    !ns_capable(net->user_ns, CAP_NET_ADMIN))
+			ret = -EPERM;
+
+		break;
+	}
+
+	genl_unlock_all();
+	return ret;
+}
+
 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,
 	};
 
 	/* we'll bump the group number right afterwards */
-- 
2.40.1