net: tap: fix wrong transport_header when sending VLAN-tagged frame

Overview

In the Linux kernel, the following vulnerability has been resolved:

net: tap: fix wrong transport_header when sending VLAN-tagged frame

In tap_get_user_xdp(), when processing a VLAN-tagged frame (e.g. ETH_P_8021Q), skb_set_network_header() is called first to advance network_header past the VLAN tag to the inner protocol header. skb_probe_transport_header() is then called with skb->protocol still set to ETH_P_8021Q, while nhoff (derived from skb_network_offset()) already points past the VLAN tag to the inner protocol header.

In __skb_flow_dissect(), proto is initialized to ETH_P_8021Q and nhoff points past the VLAN tag. When the dissector hits case ETH_P_8021Q, it reads a struct vlan_hdr at the current nhoff via __skb_header_pointer(), but that offset contains the inner protocol header (e.g. an IP header). The bytes are misinterpreted as a VLAN header, yielding a garbage encapsulated EtherType that matches no known protocol. The dissector returns false, so skb_probe_transport_header() never calls skb_set_transport_header(), leaving transport_header at its uninitialized sentinel value (~0U).

Move skb_set_network_header() to after skb_probe_transport_header(). At the time skb_probe_transport_header() is called, network_header still points to the VLAN header (offset ETH_HLEN), so nhoff is correct and the flow dissector can parse the VLAN header, extract the inner EtherType, and advance nhoff to the inner protocol header, allowing transport_header to be set correctly.

Affected

• Linux >= 8c76e77f9069f10505c08e02646c3ee11ad79038 < c0376820922d255ae1d103c32345a6ec4c3266f7
• Linux >= 8c76e77f9069f10505c08e02646c3ee11ad79038 < f1d297cad5c1eacd9fadfd0346ad97405ffbf97c
• Linux >= 8c76e77f9069f10505c08e02646c3ee11ad79038 < 5ffaa5d7f56ab24a8e23cf131eadfef31a3bbc4b
• Linux >= 8c76e77f9069f10505c08e02646c3ee11ad79038 < 88b79ac89ecc04d7f2613f7e1c0b46f0c4ddb2f3
• Linux >= 8c76e77f9069f10505c08e02646c3ee11ad79038 < cbb35cbe8db268fefe34c23df15348cf99025298
• Linux 3cae5ef1f37a475faf7c40bc6a3c170779f3e0b1
• Linux >= 4.20.1 < 4.21
• Linux 5.0

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.