Re: [PATCH net 2/2] drop_monitor: Require 'CAP_SYS_ADMIN' when joining "events" group

[Jiri Pirko <jiri@resnulli.us>]

Wed, Dec 06, 2023 at 10:31:02PM CET, idosch@nvidia.com wrote:
>The "NET_DM" generic netlink family notifies drop locations over the
>"events" multicast group. This is problematic since by default generic
>netlink allows non-root users to listen to these notifications.
>
>Fix by adding a new field to the generic netlink multicast group
>structure that when set prevents non-root users or root without the
>'CAP_SYS_ADMIN' capability (in the user namespace owning the network
>namespace) from joining the group. Set this field for the "events"
>group. Use 'CAP_SYS_ADMIN' rather than 'CAP_NET_ADMIN' because of the
>nature of the information that is shared over this group.
>
>Note that the capability check in this case will always be performed
>against the initial user namespace since the family is not netns aware
>and only operates in the initial network namespace.
>
>A new field is added to the structure rather than using the "flags"
>field because the existing field uses uAPI flags and it is inappropriate
>to add a new uAPI flag for an internal kernel check. In net-next we can
>rework the "flags" field to use internal flags and fold the new field
>into it. But for now, in order to reduce the amount of changes, add a
>new field.
>
>Since the information can only be consumed by root, mark the control
>plane operations that start and stop the tracing as root-only using the
>'GENL_ADMIN_PERM' flag.
>
>Tested using [1].
>
>Before:
>
> # capsh -- -c ./dm_repo
> # capsh --drop=cap_sys_admin -- -c ./dm_repo
>
>After:
>
> # capsh -- -c ./dm_repo
> # capsh --drop=cap_sys_admin -- -c ./dm_repo
> Failed to join "events" multicast group
>
>[1]
> $ cat dm.c
> #include <stdio.h>
> #include <netlink/genl/ctrl.h>
> #include <netlink/genl/genl.h>
> #include <netlink/socket.h>
>
> int main(int argc, char **argv)
> {
> 	struct nl_sock *sk;
> 	int grp, err;
>
> 	sk = nl_socket_alloc();
> 	if (!sk) {
> 		fprintf(stderr, "Failed to allocate socket\n");
> 		return -1;
> 	}
>
> 	err = genl_connect(sk);
> 	if (err) {
> 		fprintf(stderr, "Failed to connect socket\n");
> 		return err;
> 	}
>
> 	grp = genl_ctrl_resolve_grp(sk, "NET_DM", "events");
> 	if (grp < 0) {
> 		fprintf(stderr,
> 			"Failed to resolve \"events\" multicast group\n");
> 		return grp;
> 	}
>
> 	err = nl_socket_add_memberships(sk, grp, NFNLGRP_NONE);
> 	if (err) {
> 		fprintf(stderr, "Failed to join \"events\" multicast group\n");
> 		return err;
> 	}
>
> 	return 0;
> }
> $ gcc -I/usr/include/libnl3 -lnl-3 -lnl-genl-3 -o dm_repo dm.c
>
>Fixes: 9a8afc8d3962 ("Network Drop Monitor: Adding drop monitor implementation & Netlink protocol")
>Reported-by: "The UK's National Cyber Security Centre (NCSC)" <security@ncsc.gov.uk>
>Signed-off-by: Ido Schimmel <idosch@nvidia.com>

Reviewed-by: Jiri Pirko <jiri@nvidia.com>