DF-0359

Unguarded uint32 subtraction can underflow nr_hwavail causing self-inflicted ring-state corruption

Summary

netmap_rxsync_from_host(:906-921): n=forward distance from nr_hwcur to user-supplied cur(:917); kring->nr_hwavail -= n(:918) has no guard that n<=nr_hwavail. ring->cur is user-controlled, only bounds-checked vs k>=lim(:902) not vs nr_hwavail. Malicious user sets cur implying n>nr_hwavail -> uint32 wraps to ~4e9 -> bogus ring->avail -> host RX ring appears permanently full -> drops host-stack packets (local DoS on own netmap path). No OOB write (nm_kr_rxpos taken mod nkr_num_slots). Bounded to owning fd.

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Severity: Low
Status: new
Bucket: Base system — network stack
Confidence: likely
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U:C:N/I:N/A:L
CWE: CWE-191
File: sys/net/netmap/netmap.c
Lines: 906-921
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 04:51:41
Attempts: 1
Runtime: —
Code hash: 013eea7216c4a56a
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 04:51:41
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-0359/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0359 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	750 B	↓ raw
fix.diff	suggested-fix	fix for underflow bug	345 B	view raw

VERDICT.md verdict source verification verdict

↓ download raw

DF-0359 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: underflow

Impact: dos

Source file: sys/net/netmap/netmap.c

Mechanism

CONFIRMED: netmap_rxsync_from_host computes n from user-controlled ring->cur then does nr_hwavail-=n without guard. Unsigned underflow possible. Fix: add guard checking n<=nr_hwavail.

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/netmap/netmap.c:917.

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/netmap/netmap.c:917. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0359. No PoC binary (source-only verification).

Verified recommended fix

Add guard checking n<=nr_hwavail before subtraction. Matches finding proposal.

Verdict

CONFIRMED source-only: nr_hwavail-=n without guard, user-controlled n can cause unsigned underflow.