[PATCH net v4] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

[Anton Danilov <littlesmilingcloud@gmail.com>]

ip6gre_tunnel_xmit() and ip6erspan_tunnel_xmit() check the packet length
with pskb_inet_may_pull(), which looks at skb->protocol only, while the
code that follows parses the packet through VLAN tags with
skb_protocol(skb, true). The parsing started to look through the tags
with the two commits in the Fixes tags; the check was left as it was.
A VLAN-tagged frame whose inner IPv4/IPv6 header is not in the linear
area passes the check, and the inner header is read anyway. A 20 byte
tagged frame on ip6gretap and a 10 byte frame on ip6erspan are sent
out, while an untagged frame that is too short for its IP header is
already dropped.

Use skb_vlan_inet_prepare(), as the IPv6 receive path does since
commit 81c734dae203 ("ip6_tunnel: use skb_vlan_inet_prepare() in
__ip6_tnl_rcv()"). The second argument (inner_proto_inherit) depends on
the device: ip6gre_tunnel_xmit() serves both ip6gre (ARPHRD_IP6GRE, no
MAC header) and ip6gretap (ARPHRD_ETHER), so it is
dev->type != ARPHRD_ETHER there; ip6erspan is always an Ethernet device,
so it is false. vxlan does the same with no_eth_encap.

The tag walk starts at skb->mac_len - VLAN_HLEN, or at ETH_HLEN if
skb->mac_len is 0, and on transmit skb->mac_len is still what the skb
was received with. For a packet that came in through an NBMA gre
device and is routed out of a VLAN on ip6gretap or ip6erspan, the walk
starts inside the inner IPv4 header. Clear skb->mac_len for Ethernet
devices first.

An ip6gre device created without a remote uses ip6gre_header_ops, and
ip6gre_header() pushes a pseudo IPv6 header in front of the packet, so
there skb->data is not where the packet starts. skb_vlan_inet_prepare()
would move the network header onto that pseudo header, whose payload
length and GRE words are never written, and an ICMPv6 error for the
packet then quotes them: KMSAN reports uninit-value in
icmpv6_push_pending_frames(). Keep pskb_inet_may_pull() for that case,
as before this change.

With this change the two short frames above are dropped. gre_gso.sh,
l2_tos_ttl_inherit.sh and the mirror_gre, mirror_gre_vlan,
mirror_gre_bridge_1q and mirror_gre_changes forwarding selftests pass.
A constant true instead breaks the ip6gretap cases of mirror_gre.sh,
and a constant false breaks the ip6gre GSO cases of gre_gso.sh.

Fixes: 3f8a8447fd0b ("ip6_gre: use actual protocol to select xmit")
Fixes: b09ab9c92e50 ("ip6_tunnel: allow to inherit from VLAN encapsulated IP")
Reported-by: syzbot+6023ea32e206eef7920a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6023ea32e206eef7920a
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>

---

v4: Anton Danilov takes over the patch, as Eric suggested in the v3 thread.
 - inner_proto_inherit follows the device type instead of being a
   constant.
 - Clear skb->mac_len for Ethernet devices before the VLAN walk.
 - Keep pskb_inet_may_pull() for ip6gre with header_ops.
 - Fixes tags point at the commits that made the parsing look through
   VLAN tags, instead of d8a6213d70ac ("geneve: fix header validation
   in geneve[6]_xmit_skb"), which only added the helper.
 - Discussion: https://lore.kernel.org/netdev/20261003220513.107668-1-littlesmilingcloud@gmail.com/
v3: https://lore.kernel.org/netdev/20260119112512.28196-1-fw@strlen.de/
v2: https://lore.kernel.org/netdev/20260106144529.1424886-1-edumazet@google.com/
v1: https://lore.kernel.org/netdev/20260105100330.2258612-1-edumazet@google.com/

 net/ipv6/ip6_gre.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index e61cb10b50dc..f48141820fb5 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -883,8 +883,22 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
 	__be16 payload_protocol;
 	int ret;
 
-	if (!pskb_inet_may_pull(skb))
-		goto tx_err;
+	if (dev->type != ARPHRD_ETHER && dev->header_ops) {
+		/* ip6gre_header() has pushed a pseudo header in front of
+		 * the packet, so skb->data is not where the packet starts.
+		 */
+		if (!pskb_inet_may_pull(skb))
+			goto tx_err;
+	} else {
+		/* The VLAN tag walks below start at skb->mac_len - VLAN_HLEN,
+		 * or at ETH_HLEN if it is 0, and a forwarded skb still has
+		 * the mac_len of the device it was received on.
+		 */
+		if (dev->type == ARPHRD_ETHER)
+			skb->mac_len = 0;
+		if (skb_vlan_inet_prepare(skb, dev->type != ARPHRD_ETHER))
+			goto tx_err;
+	}
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
 		goto tx_err;
@@ -934,7 +948,12 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	__u32 mtu;
 	int nhoff;
 
-	if (!pskb_inet_may_pull(skb))
+	/* The VLAN tag walks below start at skb->mac_len - VLAN_HLEN, or at
+	 * ETH_HLEN if it is 0, and a forwarded skb still has the mac_len of
+	 * the device it was received on.
+	 */
+	skb->mac_len = 0;
+	if (skb_vlan_inet_prepare(skb, false))
 		goto tx_err;
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
-- 
2.47.3