flow_dissector: check device type before reading ETH_ADDRS

Overview

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

flow_dissector: check device type before reading ETH_ADDRS

__skb_flow_dissect() unconditionally reads 12 bytes from eth_hdr(skb) when FLOW_DISSECTOR_KEY_ETH_ADDRS is requested. This assumes the skb has a valid Ethernet header at mac_header, which is not always the case.

The problem can be triggered by:

1. Creating a TUN device in L3 mode (IFF_TUN, hard_header_len=0)
2. Attaching a multiq qdisc with a flower filter matching on eth_src
3. Sending a packet through AF_PACKET

Since TUN in L3 mode has no link-layer header, mac_header points to the L3 data area. The flow dissector reads 12 bytes of uninitialized skb memory, which then propagates through fl_set_masked_key() and is used as a rhashtable lookup key in __fl_lookup(), as reported by KMSAN.

Rejecting the filter in the control path (at tc filter add time) is not feasible because TC filter blocks can be shared between arbitrary devices -- a filter installed on an Ethernet device may later classify packets on a headerless device through a shared block. The device association is not fixed at filter creation time.

Fix this by gating the memcpy on dev->type == ARPHRD_ETHER, which ensures only true Ethernet-framed packets have their addresses read. This is more precise than the previous hard_header_len >= 12 check, which would incorrectly pass for non-Ethernet link types like IPoIB (ARPHRD_INFINIBAND, hard_header_len=24) and FDDI (hard_header_len=21) whose L2 headers are not in Ethernet format. Additionally check skb_mac_header_was_set() to guard against the pathological case where mac_header is the unset sentinel (~0U), which would cause eth_hdr() to return a wild pointer.

For the act_mirred redirect case (Ethernet packet redirected to a non-Ethernet device sharing a TC block), zeroing the key is the correct behavior: the packet is now being classified on the target device, where Ethernet address matching is not semantically meaningful.

Note: on non-Ethernet devices, the zeroed key will match a filter configured with all-zero MAC addresses. This is an improvement over the previous behavior where uninitialized memory could randomly match any filter.

Affected

• Linux >= 67a900cc0436d74e7ff89042371760def087680d < 0fe6455b8e1a22414f39c07bc90a1df12b52ec74
• Linux >= 67a900cc0436d74e7ff89042371760def087680d < 9a65860959db594dfc1820c7fdc09285fb7556bf
• Linux >= 67a900cc0436d74e7ff89042371760def087680d < 594c90b197141944f25991b8314de5c26ee27a7e
• Linux >= 67a900cc0436d74e7ff89042371760def087680d < c6d3bcb0f934d4297ac5fa1c8656ae40694fb601
• Linux >= 67a900cc0436d74e7ff89042371760def087680d < 825de39f0c35a112148799b3cbe45af3766c018a
• Linux >= 67a900cc0436d74e7ff89042371760def087680d < bf6e8af2c8be77489bedeae9f8a9654cb710e500
• Linux 4.2

Remediation

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