Re: [PATCH net-next 1/9] vxlan: initialize _md in vxlan_xmit_one()

On Thu, Sep 3, 2026 at 5:08 AM Eric Dumazet [off-list ref] wrote:

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.

Fixes: ee122c79d422 ("vxlan: Flow based tunneling")
Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>