commit e03781879a0d524ce3126678d50a80484a513c4b upstream.
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: Jacob Keller jacob.e.keller@intel.com Reviewed-by: Jiri Pirko jiri@nvidia.com Link: https://lore.kernel.org/r/20231206213102.1824398-3-idosch@nvidia.com Signed-off-by: Jakub Kicinski kuba@kernel.org --- include/net/genetlink.h | 2 ++ net/core/drop_monitor.c | 4 +++- net/netlink/genetlink.c | 3 +++ 3 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/include/net/genetlink.h b/include/net/genetlink.h index d9c4936c9941..08302fa8085b 100644 --- a/include/net/genetlink.h +++ b/include/net/genetlink.h @@ -11,10 +11,12 @@ /** * struct genl_multicast_group - generic netlink multicast group * @name: name of the multicast group, names are per-family + * @cap_sys_admin: whether %CAP_SYS_ADMIN is required for binding */ struct genl_multicast_group { char name[GENL_NAMSIZ]; u8 flags; + u8 cap_sys_admin:1; };
struct genl_ops; diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c index 006b7d72aea7..9ca28e66fabf 100644 --- a/net/core/drop_monitor.c +++ b/net/core/drop_monitor.c @@ -122,7 +122,7 @@ static struct sk_buff *reset_per_cpu_data(struct per_cpu_dm_data *data) }
static const struct genl_multicast_group dropmon_mcgrps[] = { - { .name = "events", }, + { .name = "events", .cap_sys_admin = 1 }, };
static void send_dm_alert(struct work_struct *work) @@ -370,10 +370,12 @@ static const struct genl_ops dropmon_ops[] = { { .cmd = NET_DM_CMD_START, .doit = net_dm_cmd_trace, + .flags = GENL_ADMIN_PERM, }, { .cmd = NET_DM_CMD_STOP, .doit = net_dm_cmd_trace, + .flags = GENL_ADMIN_PERM, }, };
diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c index 366206078b11..acb04573ba1d 100644 --- a/net/netlink/genetlink.c +++ b/net/netlink/genetlink.c @@ -982,6 +982,9 @@ static int genl_bind(struct net *net, int group) if ((grp->flags & GENL_UNS_ADMIN_PERM) && !ns_capable(net->user_ns, CAP_NET_ADMIN)) ret = -EPERM; + if (grp->cap_sys_admin && + !ns_capable(net->user_ns, CAP_SYS_ADMIN)) + ret = -EPERM;
break; }