DF-0358

Sign error in netmap_grab_packets: computes cur+reserved instead of cur-reserved, forwarding wrong slots to host stack

Summary

netmap_grab_packets(:736-741) comment says "compute the final position, ring->cur - ring->reserved" but code does k += n(:740) i.e. ADDS reserved instead of subtracting. Correct modular subtraction exists at netmap_rxsync_from_host(:914) k=(k>=resvd)?k-resvd:k+lim-resvd. Result: wrong range of slots forwarded to/from host stack via NIOCTXSYNC/poll host-ring path. Memory accesses stay bounded (nm_next wraps at lim, slot->len validated :748, BDG_NMB falls back to buf 0) so no OOB — wrong-packet delivery only. Device 0660 root:wheel.

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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:L/UI:N/S:U:C:N/I:L/A:L
CWE: CWE-682
File: sys/net/netmap/netmap.c
Lines: 736-741
Area: net
Discovered: 2026-07-01
Updated: 2026-07-01

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: none
Confidence: certain
Tested: 2026-07-23 04:51:41
Attempts: 1
Runtime: —
Code hash: c841559d159a77b5
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: certain
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-0358/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0358 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	705 B	↓ raw
fix.diff	suggested-fix	fix for logic bug	321 B	view raw

VERDICT.md verdict source verification verdict

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DF-0358 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: logic

Impact: none

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

Mechanism

CONFIRMED: netmap_grab_packets comment says cur-reserved but code does k+=n (adds reserved instead of subtracting). Fix: change k+=n to k-=n.

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:736.

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:736. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0358. No PoC binary (source-only verification).

Verified recommended fix

Change k+=n to k-=n in netmap_grab_packets. Matches finding proposal.

Verdict

CONFIRMED source-only: comment says cur-reserved but code adds reserved (k+=n). Fix: k-=n.