[PATCH v2 net-next 1/8] vxlan: initialize _md in vxlan_xmit_one()

[Eric Dumazet <edumazet@google.com>]

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>
---
 drivers/net/vxlan/vxlan_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index 0f0dfb4e176a98ca9a01cea6db548116ff8b0d95..34e3563362358bc4b085fdac2a7167d094feaf26 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -2368,7 +2368,7 @@ void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
 	struct ip_tunnel_key key;
 	struct vxlan_dev *vxlan = netdev_priv(dev);
 	const struct iphdr *old_iph;
-	struct vxlan_metadata _md;
+	struct vxlan_metadata _md = {};
 	struct vxlan_metadata *md = &_md;
 	unsigned int pkt_len = skb->len;
 	__be16 src_port = 0, dst_port;
-- 
2.55.0.979.g7e5102b832-goog