flow_dissector: avoid u16 truncation of hlen in bpf_flow_dissect()

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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 99bda1ecbd
("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.

Fixes: d58e468b11 ("flow_dissector: implements flow dissector BPF hook")
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20261007045616.445596-1-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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Eric Dumazet 2026-10-07 06:56:15 +02:00 • committed by Jakub Kicinski

parent a51d233ccd

commit f202578efc

1 changed files with 2 additions and 0 deletions

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net/core/flow_dissector.c

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