Linux kernel: BPF flow dissector truncates header length, misreads packet offsets

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.