Re: [PATCH v2] openvswitch: allow management from inside user namespaces

[Serge Hallyn]

Quoting Tycho Andersen (tycho.andersen@xxxxxxxxxxxxx):
> 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 introduce a new flag, GENL_UNS_ADMIN_PERM for operations
> which should be allowed inside a user namespace.
> 
> 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.
> 
> v2: use the GENL_UNS_ADMIN_PERM flag instead of a check in each function
> 
> Reported-by: James Page <james.page@xxxxxxxxxxxxx>
> Signed-off-by: Tycho Andersen <tycho.andersen@xxxxxxxxxxxxx>
> CC: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> CC: Pravin Shelar <pshelar@xxxxxxx>
> CC: Justin Pettit <jpettit@xxxxxxxxxx>
> CC: "David S. Miller" <davem@xxxxxxxxxxxxx>
> ---
>  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)) ||

Seems like this would be a lot clearer if you split it up, i.e.:

	/* CAP_NET_ADMIN required against initial user_ns */
	if ((ops->flags & GENL_ADMIN_PERM) &&
	    !netlink_capable(skb, CAP_NET_ADMIN))
		return -EPERM;

	/* CAP_NET_ADMIN required against device user_ns */
	if ((ops->flags & GENL_UNS_ADMIN_PERM) &&
	    !netlink_ns_capable(skb, net->user_ns, CAP_NET_ADMIN))
		return -EPERM;

> +	    ((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. */

Hm, I'd like to suggest adding 'over netns', but I guess that breaks 80 cols...

>  	  .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
> 
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