[PATCH net] net: sched: gred: fix 32-bit backlog wrap in gred_enqueue

gred_enqueue() admits a packet when the current backlog plus the packet
length fits within the queue limit:

  sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit   (default VQ)
  gred_backlog(t, q, sch) + qdisc_pkt_len(skb) <= q->limit (configured VQ)

sch->qstats.backlog and q->backlog are u32, and qdisc_pkt_len() returns
unsigned int, so both sums are computed in 32 bits and wrap at 2^32.
Once the true backlog exceeds 4 GiB the wrapped sum becomes small and
admission keeps succeeding, so the queue grows without bound and the
kernel can be driven to OOM.

Promote the sums to u64 so admission stops once the true backlog exceeds
the limit.  The limit is u32, so the bounded queue stays below 2^32 and
the stored u32 backlog never wraps.

Fixes: a3eb95f891d6 ("net_sched: gred: add TCA_GRED_LIMIT attribute")
Reported-by: vega@nebusec.ai
Tested-by: Victor Nogueira <redacted>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_gred.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_gred.c b/net/sched/sch_gred.c
index fcc1a4c03636..f04f425c6c44 100644
--- a/net/sched/sch_gred.c
+++ b/net/sched/sch_gred.c

@@ -179,7 +179,7 @@ static int gred_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			 * if no default DP has been configured. This
 			 * allows for DP flows to be left untouched.
 			 */
-			if (likely(sch->qstats.backlog + qdisc_pkt_len(skb) <=
+			if (likely((u64)sch->qstats.backlog + qdisc_pkt_len(skb) <=
 					sch->limit))
 				return qdisc_enqueue_tail(skb, sch);
 			else

@@ -244,7 +244,7 @@ static int gred_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		break;
 	}
 
-	if (gred_backlog(t, q, sch) + qdisc_pkt_len(skb) <= q->limit) {
+	if ((u64)gred_backlog(t, q, sch) + qdisc_pkt_len(skb) <= q->limit) {
 		q->backlog += qdisc_pkt_len(skb);
 		return qdisc_enqueue_tail(skb, sch);
 	}

-- 
2.34.1