In the Linux kernel, the following vulnerability has been resolved: vxlan: initialize _md in...

In the Linux kernel, the following vulnerability has been resolved:

vxlan: initialize _md in vxlan_xmit_one()

If a VXLAN device is configured with both VXLAN_F_COLLECT_METADATA and VXLAN_F_GBP, and a packet is transmitted through it using an external ip_tunnel_info that lacks the IP_TUNNEL_VXLAN_OPT_BIT flag, md is left pointing to the uninitialized _md stack variable:

            if (test_bit(IP_TUNNEL_VXLAN_OPT_BIT, info->key.tun_flags)) {
                    if (info->options_len < sizeof(*md))
                            goto drop;
                    md = ip_tunnel_info_opts(info);
            }

Because IP_TUNNEL_VXLAN_OPT_BIT is not set, md is not updated and remains pointing to _md. Later, vxlan_build_skb() is called with md, which eventually calls vxlan_build_gbp_hdr():

    if (vxflags & VXLAN_F_GBP)
            vxlan_build_gbp_hdr(vxh, md);

Inside vxlan_build_gbp_hdr(), md->gbp is read:

    if (!md->gbp)
            return;
    gbp = (struct vxlanhdr_gbp *)vxh;
    ...
    if (md->gbp & VXLAN_GBP_DONT_LEARN)
            gbp->dont_learn = 1;

If the stack contains garbage, this causes:

1. VXLAN_HF_GBP flag to be spuriously set in the VXLAN header.
2. gbp->dont_learn and gbp->policy_applied to be set from stack bits.
3. gbp->policy_id to receive 16 bits of uninitialized kernel stack data, leaking it onto the wire.

Fix this by zero-initializing _md. If IP_TUNNEL_VXLAN_OPT_BIT is not present, md->gbp remains 0, and vxlan_build_gbp_hdr() returns early without modifying the VXLAN header.