[PATCH 2/2] net/sched: sch_cake: check negative transport offset in cake_overhead()

[zjamg <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);

Both skb_transport_offset() and skb_inner_transport_offset() return a
signed int, but hdr_len is declared as unsigned int.

If an encapsulated or stripped packet has a negative transport offset,
hdr_len is converted to a huge positive integer near UINT_MAX. When
passed to skb_header_pointer(skb, hdr_len, ...), the signed offset argument
in skb_header_pointer() becomes negative, which can lead to an
out-of-bounds pointer (skb->data + negative_offset) and an OOB read.

Fix this by declaring hdr_len as int and falling back to the standard
cake_calc_overhead() calculation if hdr_len is negative.

Fixes: a41851bea7bf ("net: account for encap headers in qdisc pkt len")
Cc: stable@vger.kernel.org
Signed-off-by: zjamg <ndaugoing@gmail.com>
---
 net/sched/sch_cake.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index dc93267029e7..ce9e85f9571e 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1413,10 +1413,11 @@ 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));
 
@@ -1429,6 +1430,9 @@ static u32 cake_overhead(struct cake_sched_data *q, const struct sk_buff *skb)
 	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 |
 						SKB_GSO_TCPV6))) {
-- 
2.53.0