[PATCH 6.12 005/670] net: limit priority VLAN untag depth

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kamil Sawicki <user148783812897@gmail.com>

[ Upstream commit d3ff07f5acc573e5f72bf1c581bae5d05191ba61 ]

The priority VLAN receive path can repeatedly remove VID 0 tags and retry
packet processing. Unlike the read-only VLAN protocol parser, this path has
no limit on the number of nested tags it consumes.

Each skb_vlan_untag() advances skb->data and resets header offsets. The
offsets are stored in 16-bit fields. With enough nested tags, the network
or transport offset can wrap. A later protocol handler can then derive a
large pull length from the wrapped offsets and access memory outside the
skb allocation.

Apply the existing VLAN_MAX_DEPTH limit to the exceptional VID 0 retry.
Count the tag already stored in skb VLAN metadata as the first level, then
stop before untagging a ninth level. Keep the accounting inside this
uncommon branch so the regular VLAN receive path is unaffected.

A single VID 0 priority tag before a real VLAN tag is valid. No normal
deployment was found that intentionally emits multiple consecutive VID 0
tags; permitting eight levels is therefore conservative.

The issue was found while researching the Linux network receive path for
Google kvmCTF, with OpenAI Codex (gpt-5.6-sol) assisting the analysis. The
affected path, reproducer and fix were manually reviewed and tested.

Fixes: 36b2f61a42c2 ("net: handle 802.1P vlan 0 packets properly")
Cc: stable@vger.kernel.org
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Kamil Sawicki <user148783812897@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20261024004835.17862-1-user148783812897@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

[ Backport to 6.12.y: __netif_receive_skb_core() has no local
  drop_reason variable, and this branch does not define
  SKB_DROP_REASON_RECURSION_LIMIT. Omit the upstream assignment
  and keep the existing SKB_DROP_REASON_UNHANDLED_PROTO drop path. ]
Signed-off-by: Kamil Sawicki <user148783812897@gmail.com>

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/core/dev.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 397fd0f9cc7a2..058965698004d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5760,6 +5760,8 @@ static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
 	}
 
 	if (unlikely(skb_vlan_tag_present(skb)) && !netdev_uses_dsa(skb->dev)) {
+		unsigned int priority_tag_depth = 1;
+
 check_vlan_id:
 		if (skb_vlan_tag_get_id(skb)) {
 			/* Vlan id is non 0 and vlan_do_receive() above couldn't
@@ -5771,6 +5773,10 @@ static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
 			 * 802.1Q or 802.1AD and vlan_do_receive() above could
 			 * not find vlan dev for vlan id 0.
 			 */
+			if (unlikely(priority_tag_depth >= VLAN_MAX_DEPTH))
+				goto drop;
+
+			priority_tag_depth++;
 			__vlan_hwaccel_clear_tag(skb);
 			skb = skb_vlan_untag(skb);
 			if (unlikely(!skb))
-- 
2.53.0