In the Linux kernel, the following vulnerability has been resolved: flow_dissector: avoid u16 truncation of hlen in bpf_flow_dissect() bpf

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.