[PATCH] net/sched: sch_netem: prevent packet length underflow with negative overhead

[Bui Viet Dung <dungvn2345@gmail.com>]

packet_time_ns() calculates transmission time for netem rate shaping.
It adds q->packet_overhead (signed 32-bit int) to len (unsigned
64-bit int).

When userspace configures a negative packet overhead (via
TCA_NETEM_RATE packet_overhead attribute) and an enqueued packet's
length is smaller than the absolute overhead, (s64)len +
q->packet_overhead is negative. Because len is u64, the addition wraps
into an astronomical value (~2^64 - 1). When multiplied by NSEC_PER_SEC
and divided by q->rate, the calculated delay spans hours or days,
causing enqueued packets to be frozen in the qdisc indefinitely and
stalling transmission.

A similar underflow can occur in cell length calculation when
(q->cell_size + q->cell_overhead) is non-positive.

Clamp the effective packet length to 0 if the overhead adjustment
would underflow, and return 0 delay if effective cell length is
non-positive, matching the behavior in sch_cake and sch_tbf.

Fixes: 7bc0f28c7a0c ("netem: rate extension")
Cc: stable@vger.kernel.org
Signed-off-by: Bui Viet Dung <dungvn2345@gmail.com>
---
 net/sched/sch_netem.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 8a1b89a289d..a3dcd33bec4 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -366,14 +366,20 @@ static s64 tabledist(s64 mu, s32 sigma,
 
 static u64 packet_time_ns(u64 len, const struct netem_sched_data *q)
 {
+	if ((s64)len + q->packet_overhead <= 0)
+		return 0;
+
 	len += q->packet_overhead;
 
 	if (q->cell_size) {
 		u32 cells = reciprocal_divide(len, q->cell_size_reciprocal);
+		s32 cell_len = (s32)q->cell_size + q->cell_overhead;
 
 		if (len > cells * q->cell_size)	/* extra cell needed for remainder */
 			cells++;
-		len = cells * (q->cell_size + q->cell_overhead);
+		if (cell_len <= 0)
+			return 0;
+		len = (u64)cells * cell_len;
 	}
 
 	return div64_u64(len * NSEC_PER_SEC, q->rate);
-- 
2.43.0