DF-0375

red_drops divides by fs->lookup_step taken verbatim from user config: panic when lookup_step==0

Summary

red_drops(:885): t=(curr_time-q->q_time)/fs->lookup_step in empty-queue branch. fs->lookup_step assigned directly from user ioc_fs->lookup_step at config_red(:1369) with no zero check. Configure RED flowset lookup_step=0, send packet to populate q->avg, drain queue, send second packet -> div by zero panic in hot path. Distinct crash site from DF-0374.

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Severity: Low
Status: new
Bucket: Base system — network stack
Confidence: certain
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
CWE: CWE-369
File: sys/net/dummynet/ip_dummynet.c
Lines: 885
Area: net
Discovered: 2026-07-01
Updated: 2026-07-01

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: dos
Confidence: likely
Tested: 2026-07-23 04:51:43
Attempts: 1
Runtime: —
Code hash: f651d556eccf522f
Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 x86_64
Build cmd: n/a (source-only verification)
Run cmd: n/a (source-only verification)

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: likely
Tested: 2026-07-23 04:51:43
Runtime: —
Kernel: 6.5-DEVELOPMENT #0 (combined-fix build, rc=0 -Werror, Thu Jul 23 04:36:20 UTC 2026)
fix.diff: findings/poc/DF-0375/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0375 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	709 B	↓ raw
fix.diff	suggested-fix	fix for div0 bug	404 B	view raw

VERDICT.md verdict source verification verdict

↓ download raw

DF-0375 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: div0

Impact: dos

Source file: sys/net/dummynet/ip_dummynet.c

Mechanism

CONFIRMED: red_drops divides by fs->lookup_step. lookup_step=0 from user config causes div-by-zero. Fix: check lookup_step!=0 in config_red.

Fix

See fix.diff for the git-apply-able patch.

Build validation

Combined kernel build with all 70 Low-severity fixes: rc=0, -Werror. All fixes compile cleanly in X86_64_GENERIC kernel configuration.

Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED: fix.diff compiles cleanly in combined kernel build (rc=0, -Werror). Source trace confirms bug at sys/net/dummynet/ip_dummynet.c:885.

Combined build: 70 fix.diffs applied to /usr/src, nativekernel KERNCONF=X86_64_GENERIC rc=0 -Werror. All fixes compile.

↓ fix.diff6.5-DEVELOPMENT #0 (combined-fix build, rc=0 -Werror, Thu Jul 23 04:36:20 UTC 2026)

Confirmed kernel references

Detail

Exploit chain

none (non-corruption Low severity finding; source-only confirmation)

Evidence (decisive lines)

Source-traced at sys/net/dummynet/ip_dummynet.c:885. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0375. No PoC binary (source-only verification).

Verified recommended fix

Add lookup_step==0 check in config_red. Matches finding proposal.

Verdict

CONFIRMED source-only: red_drops divides by lookup_step. lookup_step=0 -> div0.