Listening for netlink broadcasts in a kernel module

Question

The SELinux module sends out a netlink broadcast to any listening sockets. I'm wondering if it's possible to listen for netlink broadcast from within another kernel module?

From SELinux netlink code:

netlink_broadcast(selnl, skb, 0, SELNLGRP_AVC, GFP_USER);

Answer 1

I found that you can listen for netlink data through the use of regular sockets. And, yes, it's possible in kernel-space.

You basically need to create and bind to a socket:

struct sock *sock = NULL;
struct sockaddr_nl addr = { 0 };

/* Create a netlink socket for SELinux traffic */
int rc = sock_create_kern(AF_NETLINK, SOCK_RAW, NETLINK_SELINUX,
              &ctx.sock);

if (rc)
    return rc;

addr.nl_family  = AF_NETLINK;
addr.nl_pid     = 0;
addr.nl_groups  = SELNLGRP_AVC;

rc = kernel_bind(ctx.sock, (struct sockaddr *) &addr, sizeof(addr));
if (rc)
    return rc;

/* Setup socket callback */
sock = ctx.sock->sk;
sock->sk_data_ready = netlink_data_ready;
sock->sk_allocation = GFP_KERNEL;

To receive the data:

static void netlink_data_ready(struct sock *sk, int bytes)
{
    struct sk_buff *skb = NULL;
    struct nlmsghdr *nlh = NULL;
    int rc = 0;

    /* Receive the data packet (blocking) */
    skb = skb_recv_datagram(sk, 0, 0, &rc);
    if (rc) {
        printk(KERN_ERROR "Failed on skb_recv_datagram(). rc=%d.", -rc);
        return;
    }

    nlh = (struct nlmsghdr *) skb->data;
    if (!nlh || !NLMSG_OK(nlh, bytes)) {
        printk(KERN_ERROR "Invalid netlink header data.");
        return;
    }

    if (nlh->nlmsg_type == SELNL_MSG_POLICYLOAD ||
        nlh->nlmsg_type == SELNL_MSG_SETENFORCE) {

        /* Insert code here */
    }
}