[PATCH net] flow_dissector: avoid u16 truncation of hlen in bpf_flow_dissect()

[Eric Dumazet <edumazet@kernel.org>]

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.

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Assisted-by: LLM
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/core/flow_dissector.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 27d8a01bc92306ff043389b4fde2d24af97d3106..1be6c739e3c718b4561195a20e49cc3911874af1 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1023,6 +1023,8 @@ u32 bpf_flow_dissect(struct bpf_prog *prog, struct bpf_flow_dissector *ctx,
 
 	result = bpf_prog_run_pin_on_cpu(prog, ctx);
 
+	/* bpf_flow_keys offsets are u16: do not let @hlen be truncated. */
+	hlen = min_t(int, hlen, U16_MAX);
 	flow_keys->nhoff = clamp_t(u16, flow_keys->nhoff, nhoff, hlen);
 	flow_keys->thoff = clamp_t(u16, flow_keys->thoff,
 				   flow_keys->nhoff, hlen);
-- 
2.53.0