[PATCH net v2] net/sched: sch_netem: prevent packet length underflow with negative overhead

[PATCH net v2] net/sched: sch_netem: prevent packet length underflow with negative overhead

[Bui Viet Dung]

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

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 non-positive. 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 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.

Furthermore, computing cell_len in s64 prevents sign corruption if
cell_size >= 0x80000000 or if cell_size + cell_overhead exceeds INT_MAX.
Computing the final transmission time with mul_u64_u64_div_u64() and
clamping to S64_MAX avoids 64-bit multiplication overflow and prevents
the resulting delay from turning negative in s64 delay arithmetic.

Return 0 delay if effective packet length or cell length is non-positive
so that non-positive effective packet lengths experience no rate
throttling rather than freezing transmission.

The issue was discovered via manual code audit of sch_netem.c and
verified on a test VM using 'tc qdisc add dev lo root netem rate 100Mbit
overhead -100', which caused immediate backlog stalls on small packets.

Fixes: 7bc0f28c7a0c ("netem: rate extension")
Cc: stable@vger.kernel.org
Signed-off-by: Bui Viet Dung <dungvn2345@gmail.com>
---
Link to v1: https://lore.kernel.org/netdev/20261008035656.330667-1-dungvn2345@gmail.com/

v1 -> v2:
- Calculate cell_len as s64 to avoid sign corruption when cell_size >= 0x80000000 or upon s32 overflow (Sashiko AI review).
- Use mul_u64_u64_div_u64() to prevent u64 multiplication overflow with large cell configurations, and clamp result to S64_MAX to prevent negative s64 delays (Sashiko AI review).
- Reword commit message: remove inaccurate comparisons to sch_cake and sch_tbf; clarify the clamping policy for non-positive effective length (Sashiko AI review).
- Include <linux/math64.h>.

 net/sched/sch_netem.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 8a1b89a289d8..3eccdc93c0b6 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -21,6 +21,7 @@
 #include <linux/rtnetlink.h>
 #include <linux/reciprocal_div.h>
 #include <linux/rbtree.h>
+#include <linux/math64.h>
 
 #include <net/gso.h>
 #include <net/netlink.h>
@@ -366,17 +367,26 @@ static s64 tabledist(s64 mu, s32 sigma,
 
 static u64 packet_time_ns(u64 len, const struct netem_sched_data *q)
 {
+	u64 d;
+
+	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);
+		s64 cell_len = (s64)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);
+	d = mul_u64_u64_div_u64(len, NSEC_PER_SEC, q->rate);
+	return min_t(u64, d, S64_MAX);
 }
 
 static void tfifo_reset(struct Qdisc *sch)
-- 
2.43.0

[syzbot ci] Re: net/sched: sch_netem: prevent packet length underflow with negative overhead

[syzbot ci]

syzbot ci has tested the following series

[v2] net/sched: sch_netem: prevent packet length underflow with negative overhead
https://lore.kernel.org/all/20261012044322.503949-1-dungvn2345@gmail.com
* [PATCH net v2] net/sched: sch_netem: prevent packet length underflow with negative overhead

and found the following issue:
divide error in netem_enqueue

Full report is available here:
https://ci.syzbot.org/series/eff5cc04-13f7-4a28-9b83-b6e0de530bc2

***

divide error in netem_enqueue

tree:      net
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git
base:      af32da41b0327b9c6a37856ba82b6760d6c8d10e
arch:      amd64
compiler:  Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
config:    https://ci.syzbot.org/builds/bf12d8a1-2cda-46fc-9150-cadd809b1c71/config

Oops: divide error: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 17390 Comm: syz.0.2912 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:netem_enqueue+0x20f6/0x3300
Code: 8b 74 24 38 48 8b 44 24 50 42 80 3c 28 00 74 08 4c 89 f7 e8 cc f4 51 f8 49 8b 0e 4c 89 f8 48 f7 25 b7 9c 8b 04 48 89 4c 24 78 <48> f7 74 24 78 48 b9 ff ff ff ff ff ff ff 7f 48 39 c8 48 0f 42 c8
RSP: 0018:ffffc900082cf4b0 EFLAGS: 00010a07
RAX: 3599740d73380000 RBX: 0000005c44b22572 RCX: 000000000000002c
RDX: 0000000000000077 RSI: 00000000000001ed RDI: 00000000000001ee
RBP: ffff88810e973000 R08: ffffffff917a717f R09: 1ffffffff22f4e2f
R10: dffffc0000000000 R11: ffffffff81776d40 R12: ffff888026e62330
R13: dffffc0000000000 R14: ffff88810e9732b0 R15: 0000020000176c00
FS:  00007f87167196c0(0000) GS:ffff88818c4bf000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f8716718ff8 CR3: 000000002756e000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 netem_enqueue+0x16f1/0x3300
 dev_qdisc_enqueue+0x4e/0x260
 __dev_queue_xmit+0x1b6c/0x3920
 packet_sendmsg+0x3f11/0x51a0
 sock_sendmsg_nosec+0x13a/0x180
 __sys_sendto+0x408/0x5a0
 __x64_sys_sendto+0xde/0x100
 do_syscall_64+0x166/0x520
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f871579e1d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f8716719028 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f8715a25fa0 RCX: 00007f871579e1d9
RDX: 00000000000005dc RSI: 0000200000000280 RDI: 0000000000000009
RBP: 00007f8715835888 R08: 0000200000000140 R09: 0000000000000014
R10: 0000000000004009 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8715a26038 R14: 00007f8715a25fa0 R15: 00007ffc85273198
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:netem_enqueue+0x20f6/0x3300
Code: 8b 74 24 38 48 8b 44 24 50 42 80 3c 28 00 74 08 4c 89 f7 e8 cc f4 51 f8 49 8b 0e 4c 89 f8 48 f7 25 b7 9c 8b 04 48 89 4c 24 78 <48> f7 74 24 78 48 b9 ff ff ff ff ff ff ff 7f 48 39 c8 48 0f 42 c8
RSP: 0018:ffffc900082cf4b0 EFLAGS: 00010a07
RAX: 3599740d73380000 RBX: 0000005c44b22572 RCX: 000000000000002c
RDX: 0000000000000077 RSI: 00000000000001ed RDI: 00000000000001ee
RBP: ffff88810e973000 R08: ffffffff917a717f R09: 1ffffffff22f4e2f
R10: dffffc0000000000 R11: ffffffff81776d40 R12: ffff888026e62330
R13: dffffc0000000000 R14: ffff88810e9732b0 R15: 0000020000176c00
FS:  00007f87167196c0(0000) GS:ffff88818c4bf000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f8716718ff8 CR3: 000000002756e000 CR4: 00000000000006f0


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.

To test a fix for this bug, please reply with `#syz test`
(on a separate line) and attach the patch to the email.

Notes:
- The patch will be applied on top of the tested series (as an
  incremental fix).
- To test a new version of the whole series, please send it directly
  to syzbot@lists.linux.dev.
- Arguments like custom git repos and branches are not supported.