flow_dissector: avoid u16 truncation of hlen in bpf_flow_dissect()

Overview

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

flow_dissector: avoid u16 truncation of hlen in bpf_flow_dissect()

bpf_flow_dissect() sanitizes the offsets returned by the BPF program with clamp_t(u16, ..., hlen).

If hlen is bigger than 65535, the upper bound is truncated before the comparison. For instance, with hlen == 65536, both nhoff and thoff are forced to zero, even if the program returned sane values.

This is the same class of bug as the one fixed in commit 99bda1ecbd56 ("flow_dissector: avoid u16 truncation of skb->len when computing thoff"). That fix does not cover the BPF path, because __skb_flow_dissect() returns early when a BPF flow dissector handles the packet.

struct bpf_flow_keys stores nhoff and thoff as u16, so cap hlen to U16_MAX before clamping.

Found by sashiko while reviewing the commit above.

Affected

• Linux >= d58e468b1112dcd1d5193c0a89ff9f98b5a3e8b9 < c44c02c930c6583b235a1937c69ff716c4bb4514
• Linux >= d58e468b1112dcd1d5193c0a89ff9f98b5a3e8b9 < 3fb5ad2951a6df92c98f4f06cd70f20c50a2eb2a
• Linux >= d58e468b1112dcd1d5193c0a89ff9f98b5a3e8b9 < cc18eb0e9941162c03f03f411ed7f6c339d91fa0
• Linux >= d58e468b1112dcd1d5193c0a89ff9f98b5a3e8b9 < 61b43a51a55929ec040bee3ea51af2121b956d05
• Linux >= d58e468b1112dcd1d5193c0a89ff9f98b5a3e8b9 < f202578efc738b883cfa04889eb97328914265a7
• Linux 4.20

Remediation

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