CVE-2026-98377vlan: require the MAC header to be present in __vlan_insert_inner_tag()

Official description

In the Linux kernel, the following vulnerability has been resolved: vlan: require the MAC header to be present in __vlan_insert_inner_tag() __vlan_insert_inner_tag() only guarantees head room via skb_cow_head(), never that mac_len bytes of MAC header are present. Its ETH_HLEN wrappers - __vlan_insert_tag() under skb_vlan_push(), and vlan_insert_tag() under validate_xmit_vlan() on the generic transmit path - therefore rewrite the first 16 bytes at skb->data: a 12-byte memmove plus two 2-byte stores at +12 and +14. No caller supplies the bound, while the pop helpers use skb_ensure_writable()/pskb_may_pull(). An IFF_TUN device has hard_header_len == 0, so packet_snd() accepts a one-byte AF_PACKET/SOCK_RAW frame. The first vlan push only sets a hwaccel tag; the next - clsact "action vlan push" or bpf_skb_vlan_push() - enters the helper with skb->len still 1. The head comes from skbuff_small_head without __GFP_ZERO, so each push drags bytes from beyond skb->tail into the frame. After three the one-byte send leaves as 13 bytes carrying 11 bytes of uninitialised slab: 0000: 5a b3 62 12 80 88 ff ff 00 b3 62 12 81 `------------------------------' only 0x5a was sent; the rest is slab, here the top 56 bits of a linear-map address Require the MAC header the helper rewrites to be present, so such a frame is dropped rather than transmitted.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/12295e0728488105b152b57fe8bc5e86f1d21a73
• https://git.kernel.org/stable/c/aecace4ef31bd4e8ca1f224280ab52a457b0dc9a
• https://git.kernel.org/stable/c/50a958f865173e8752ff07bf0eff7b55cdeb5d74
• https://git.kernel.org/stable/c/3d531429524df2c7116579306640283fea68bcf0
• https://git.kernel.org/stable/c/40a5cc4b7251c74f3341332a226d02200e96bccf
• https://git.kernel.org/stable/c/f42562dd027dc4ed103fae17b2206e73ea1963c4
• https://git.kernel.org/stable/c/59af43ccece4d2d8b62e9e3ccc96b6e2e793bcdc
• https://git.kernel.org/stable/c/ab888242fce4f16f6c4d4c6ec53939ad36aa3b3a