[PATCH v3 1/2] wifi: nl80211: require CAP_NET_ADMIN over the target netns in SET_WIPHY_NETNS

[Maoyi Xie <maoyixie.tju@gmail.com>]

From: Maoyi Xie <maoyi.xie@ntu.edu.sg>

NL80211_CMD_SET_WIPHY_NETNS dispatches with GENL_UNS_ADMIN_PERM, which
verifies that the caller has CAP_NET_ADMIN over the user namespace
owning the source netns (the netlink socket's netns). It does not
verify that the caller has CAP_NET_ADMIN over the target netns
selected by NL80211_ATTR_NETNS_FD or NL80211_ATTR_PID.

This diverges from the convention enforced in
net/core/rtnetlink.c::rtnl_get_net_ns_capable():

    /* For now, the caller is required to have CAP_NET_ADMIN in
     * the user namespace owning the target net ns.
     */
    if (!sk_ns_capable(sk, net->user_ns, CAP_NET_ADMIN))
        return ERR_PTR(-EACCES);

A user with CAP_NET_ADMIN in their own user namespace can therefore
push a wiphy into an arbitrary netns (including init_net) over which
they have no privilege.

Reachable from an unprivileged user namespace as soon as the caller
holds, in their own netns, a wiphy that has WIPHY_FLAG_NETNS_OK set
(true for mac80211_hwsim and for any wiphy that an administrator has
delegated into a container).

Reproducer (mac80211_hwsim, KASAN VM):

    1. As real root, modprobe mac80211_hwsim radios=1 in init_net.
    2. fork(); child unshare(CLONE_NEWUSER | CLONE_NEWNET) and writes
       0-mapped uid_map.
    3. Real root migrates phyN into the child's netns via
       NL80211_CMD_SET_WIPHY_NETNS (legitimate admin step).
    4. Child, with CAP_NET_ADMIN only in its own user_ns, sends
       NL80211_CMD_SET_WIPHY_NETNS targeting init_net's netns fd.
    5. The kernel honours the request and the wiphy is moved back
       to init_net even though the caller has no privilege there.

Mirror the rtnetlink convention by requiring ns_capable(net->user_ns,
CAP_NET_ADMIN) on the resolved target netns before calling
cfg80211_switch_netns().

Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
---
 net/wireless/nl80211.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/net/wireless/nl80211.c b/net/wireless/nl80211.c
index 67088804d..db546dd93 100644
--- a/net/wireless/nl80211.c
+++ b/net/wireless/nl80211.c
@@ -13867,6 +13867,19 @@ static int nl80211_wiphy_netns(struct sk_buff *skb, struct genl_info *info)
 	if (IS_ERR(net))
 		return PTR_ERR(net);
 
+	/*
+	 * The caller already has CAP_NET_ADMIN over the source netns
+	 * (enforced by GENL_UNS_ADMIN_PERM on the genl op). Mirror the
+	 * convention used by net/core/rtnetlink.c::rtnl_get_net_ns_capable()
+	 * and require CAP_NET_ADMIN over the target netns as well, so that
+	 * a caller that is privileged in their own user namespace cannot
+	 * push a wiphy into a netns where they have no privilege.
+	 */
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN)) {
+		put_net(net);
+		return -EPERM;
+	}
+
 	err = 0;
 
 	/* check if anything to do */
-- 
2.34.1