[PATCH v2] net/sched: sch_cake: prevent shaper corruption and stall in cake_overhead()

[Yuchao Zhang <ndaugoing@gmail.com>]

In cake_overhead(), the header length up to the transport layer is
computed using logic borrowed from qdisc_pkt_len_segs_init():

        /* borrowed from qdisc_pkt_len_segs_init() */
        if (!skb->encapsulation)
                hdr_len = skb_transport_offset(skb);
        else
                hdr_len = skb_inner_transport_offset(skb);

However, cake_overhead() suffers from several issues:

1. When segs == 0 (e.g. from dodgy GSO packets where gso_segs is not
   recomputed), the multi-segment arithmetic underflows: (segs - 1) wraps
   to 4294967295, causing cake_calc_overhead() * (segs - 1) to produce
   an enormous length. When charged in cake_advance_shaper(),
   time_next_packet is pushed decades into the future, permanently
   stalling the CAKE dequeue queue.
   Fix this by returning cake_calc_overhead(q, len, off) for segs <= 1.

2. When the transport header was never set, skb->transport_header holds
   the sentinel value ~0U. skb_transport_offset() returns ~65535, which
   is not caught by a negative offset check. skb_header_pointer() fails,
   and hdr_len stays ~65535, charging ~66 KB per segment to the shaper.
   Fix this by mirroring qdisc_pkt_len_segs_init() and checking
   unlikely(!skb_transport_header_was_set(skb)).

3. Both skb_transport_offset() and skb_inner_transport_offset() return
   signed int, but hdr_len is declared as unsigned int. If post-enqueue
   mangling (such as BPF packet trimming via bpf_skb_net_shrink())
   produces a negative offset, hdr_len wraps to near UINT_MAX, causing
   corrupted header length accounting.
   Fix this by declaring hdr_len as int and falling back to
   cake_calc_overhead(q, len, off) if hdr_len is negative.

Fixes: a41851bea7bf ("net: account for encap headers in qdisc pkt len")
Cc: stable@vger.kernel.org
Signed-off-by: Yuchao Zhang <ndaugoing@gmail.com>
---
v2:
 - Accurately describe the impact as shaper accounting corruption / stall
   rather than OOB read past the allocation per Sashiko review.
 - Fix pre-existing high-severity integer underflow when segs == 0 by
   checking segs <= 1.
 - Import companion check !skb_transport_header_was_set(skb) from
   qdisc_pkt_len_segs_init() to prevent unset transport header sentinel
   (~0U) from inflating packet length to ~66 KB.

 net/sched/sch_cake.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index dc93267029e7..45969c1b95fc 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1413,21 +1413,28 @@ static u32 cake_calc_overhead(struct cake_sched_data *qd, u32 len, u32 off)
 static u32 cake_overhead(struct cake_sched_data *q, const struct sk_buff *skb)
 {
 	const struct skb_shared_info *shinfo = skb_shinfo(skb);
-	unsigned int hdr_len, last_len = 0;
+	unsigned int last_len = 0;
 	u32 off = skb_network_offset(skb);
 	u16 segs = qdisc_pkt_segs(skb);
 	u32 len = qdisc_pkt_len(skb);
+	int hdr_len;
 
 	WRITE_ONCE(q->avg_netoff, cake_ewma(q->avg_netoff, off << 16, 8));
 
-	if (segs == 1)
+	if (segs <= 1)
 		return cake_calc_overhead(q, len, off);
 
 	/* borrowed from qdisc_pkt_len_segs_init() */
-	if (!skb->encapsulation)
+	if (!skb->encapsulation) {
+		if (unlikely(!skb_transport_header_was_set(skb)))
+			return cake_calc_overhead(q, len, off);
 		hdr_len = skb_transport_offset(skb);
-	else
+	} else {
 		hdr_len = skb_inner_transport_offset(skb);
+	}
+
+	if (unlikely(hdr_len < 0))
+		return cake_calc_overhead(q, len, off);
 
 	/* + transport layer */
 	if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 |
-- 
2.53.0