Re: [PATCH] openvswitch: allow management from inside user namespaces

[Tycho Andersen]

Hi Eric,

Thanks for the review.

On Fri, Jan 29, 2016 at 08:29:55AM -0600, Eric W. Biederman wrote:
> Tycho Andersen <tycho.andersen@xxxxxxxxxxxxx> writes:
> 
> > Operations with the GENL_ADMIN_PERM flag fail permissions checks because
> > this flag means we call netlink_capable, which uses the init user ns.
> >
> > Instead, let's do permissions checks in each function, but use the netlink
> > socket's user ns instead of the initial one, to allow management of
> > openvswitch resources from inside a user ns.
> >
> > The motivation for this is to be able to run openvswitch in unprivileged
> > containers. I've tested this and it seems to work, but I really have no
> > idea about the security consequences of this patch, so thoughts would be
> > much appreciated.
> 
> So at a quick look using ns_capable this way is probably buggy.
> 
> netlink is goofy (because historically we got this wrong), and I forget
> what the specific rules are.  The general rule is that you need to do
> your permission checks on open/create/connect and not inside send/write
> while processing data.  Otherwise there is a class of privileged
> applications where you can set their stdout to some precreated file
> descriptor and their output can be made to act as a command, bypassing
> your permission checks.

It's worth noting that this patch doesn't move the checks (i.e., they
are still done at write time currently in the kernel), it just relaxes
them to root in the user ns instead of real root. This means I can
currently exploit netlink this way as an unprivileged, just not from
within an unprivileged container.

An alternate version of this patch below might be more favorable, as
we may want to do something like this elsewhere in netlink. I think it
also clarifies the situation a bit, at the cost of adding another
flag.

A third option would be to move this check to connect time, but that
would force everything in the family (since that's the only thing you
connect /to/ in netlink) to have the same required permissions, which
might (probably?) break stuff; e.g. you can call OVS_FLOW_CMD_GET
without CAP_NET_ADMIN, but if we changed everything in the family to
require it, that would break.

Tycho
---
 include/uapi/linux/genetlink.h |  1 +
 net/netlink/genetlink.c        |  6 ++++--
 net/openvswitch/datapath.c     | 20 ++++++++++----------
 3 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/include/uapi/linux/genetlink.h b/include/uapi/linux/genetlink.h
index c3363ba..5512c90 100644
--- a/include/uapi/linux/genetlink.h
+++ b/include/uapi/linux/genetlink.h
@@ -21,6 +21,7 @@ struct genlmsghdr {
 #define GENL_CMD_CAP_DO		0x02
 #define GENL_CMD_CAP_DUMP	0x04
 #define GENL_CMD_CAP_HASPOL	0x08
+#define GENL_UNS_ADMIN_PERM	0x10
 
 /*
  * List of reserved static generic netlink identifiers:
diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index f830326..6bbb3eb 100644
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -576,8 +576,10 @@ static int genl_family_rcv_msg(struct genl_family *family,
 	if (ops == NULL)
 		return -EOPNOTSUPP;
 
-	if ((ops->flags & GENL_ADMIN_PERM) &&
-	    !netlink_capable(skb, CAP_NET_ADMIN))
+	if (((ops->flags & GENL_ADMIN_PERM) &&
+	    !netlink_capable(skb, CAP_NET_ADMIN)) ||
+	    ((ops->flags & GENL_UNS_ADMIN_PERM) &&
+	    !netlink_ns_capable(skb, net->user_ns, CAP_NET_ADMIN)))
 		return -EPERM;
 
 	if ((nlh->nlmsg_flags & NLM_F_DUMP) == NLM_F_DUMP) {
diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index deadfda..d6f7fe9 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -654,7 +654,7 @@ static const struct nla_policy packet_policy[OVS_PACKET_ATTR_MAX + 1] = {
 
 static const struct genl_ops dp_packet_genl_ops[] = {
 	{ .cmd = OVS_PACKET_CMD_EXECUTE,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = packet_policy,
 	  .doit = ovs_packet_cmd_execute
 	}
@@ -1391,12 +1391,12 @@ static const struct nla_policy flow_policy[OVS_FLOW_ATTR_MAX + 1] = {
 
 static const struct genl_ops dp_flow_genl_ops[] = {
 	{ .cmd = OVS_FLOW_CMD_NEW,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = flow_policy,
 	  .doit = ovs_flow_cmd_new
 	},
 	{ .cmd = OVS_FLOW_CMD_DEL,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = flow_policy,
 	  .doit = ovs_flow_cmd_del
 	},
@@ -1407,7 +1407,7 @@ static const struct genl_ops dp_flow_genl_ops[] = {
 	  .dumpit = ovs_flow_cmd_dump
 	},
 	{ .cmd = OVS_FLOW_CMD_SET,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = flow_policy,
 	  .doit = ovs_flow_cmd_set,
 	},
@@ -1777,12 +1777,12 @@ static const struct nla_policy datapath_policy[OVS_DP_ATTR_MAX + 1] = {
 
 static const struct genl_ops dp_datapath_genl_ops[] = {
 	{ .cmd = OVS_DP_CMD_NEW,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = datapath_policy,
 	  .doit = ovs_dp_cmd_new
 	},
 	{ .cmd = OVS_DP_CMD_DEL,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = datapath_policy,
 	  .doit = ovs_dp_cmd_del
 	},
@@ -1793,7 +1793,7 @@ static const struct genl_ops dp_datapath_genl_ops[] = {
 	  .dumpit = ovs_dp_cmd_dump
 	},
 	{ .cmd = OVS_DP_CMD_SET,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = datapath_policy,
 	  .doit = ovs_dp_cmd_set,
 	},
@@ -2158,12 +2158,12 @@ static const struct nla_policy vport_policy[OVS_VPORT_ATTR_MAX + 1] = {
 
 static const struct genl_ops dp_vport_genl_ops[] = {
 	{ .cmd = OVS_VPORT_CMD_NEW,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = vport_policy,
 	  .doit = ovs_vport_cmd_new
 	},
 	{ .cmd = OVS_VPORT_CMD_DEL,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = vport_policy,
 	  .doit = ovs_vport_cmd_del
 	},
@@ -2174,7 +2174,7 @@ static const struct genl_ops dp_vport_genl_ops[] = {
 	  .dumpit = ovs_vport_cmd_dump
 	},
 	{ .cmd = OVS_VPORT_CMD_SET,
-	  .flags = GENL_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
+	  .flags = GENL_UNS_ADMIN_PERM, /* Requires CAP_NET_ADMIN privilege. */
 	  .policy = vport_policy,
 	  .doit = ovs_vport_cmd_set,
 	},
-- 
2.5.0