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DF-3274

Use-after-free read in TX accounting: m->m_pkthdr.len read after m_freem(m) when the control queue is full

Summary

sppp_cisco_send (:1310-1318), sppp_cp_send (:1367-1376) and sppp_auth_send (:4678-4687) all do: if (IF_QFULL(&sp->pp_cpq)) { m_freem(m); } else IF_ENQUEUE(...); followed unconditionally by IFNET_STAT_INC(ifp, obytes, m->m_pkthdr.len + 3) - reading 4 bytes of the freed mbuf header when the queue-full branch ran. A peer can fill pp_cpq (20 deep) and force any control transmit into the freed-mbuf read; the value feeds only the obytes counter - stale-value read, no disclosed output. Not verified (Low; hardening). Fix: capture the length before the free (all three sites). [VERIFIED pass2: reproduced via guard-page harness - all three sppp send sites read m_pkthdr.len after m_freem on the QFULL branch; counter-only impact; capture-before-free fix A/B]

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PoC verification

Evidence pack

findings/poc/DF-3274 · 13 files
FileTypeDescriptionSize
README.md — 876 B ↓ raw
VERDICT.md — 2.2 KB ↓ raw
allruns.log — 4.9 KB view raw
build.sh — 708 B view raw
df3274.c — 2.6 KB view raw
fix.diff — 487 B view raw
fixbuild.sh — 394 B view raw
fixrun.sh — 1.1 KB view raw
fixruns.log — 810 B view raw
guard.h — 1.3 KB view raw
manifest.json — 907 B view raw
run.sh — 2.0 KB view raw
verdict.json — 1.6 KB view raw
README.md
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DF-3274 -- verification pack

Method: Verbatim-interposition harness (guard-page technique; no wlan/sppp hardware on this guest) Status: REPRODUCED | impact: none | confidence: certain

What this is

UAF read of the freed mbuf pkthdr (obytes accounting, all three sppp send sites); live sppp unreachable on guest (no standalone cloner: ifconfig sppp0 create -> EINVAL), verbatim-code guard-page harness faults deterministically; value feeds only the obytes counter (netstat -i).

Build

./build.sh

Run

df3274 uaf

(success criterion: see VERDICT.md)

Evidence

control (read-before-free order) rc=0; stock order (free-then-read) faults the guard page rc=139; FIXED build clean (fixruns.log DF-3274 FIXED-OK)

Kernel references: sys/net/sppp/if_spppsubr.c:1372-1380, sys/net/sppp/if_spppsubr.c:1429-1438, sys/net/sppp/if_spppsubr.c:4672-4681

VERDICT.md
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DF-3274 -- VERDICT

REPRODUCED (impact=none, confidence=certain) 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

Summary

UAF read of the freed mbuf pkthdr (obytes accounting, all three sppp send sites); live sppp unreachable on guest (no standalone cloner: ifconfig sppp0 create -> EINVAL), verbatim-code guard-page harness faults deterministically; value feeds only the obytes counter (netstat -i).

How it was verified

  • Method: Verbatim-interposition harness (guard-page technique; no wlan/sppp hardware on this guest)
  • Kernel references: sys/net/sppp/if_spppsubr.c:1372-1380, sys/net/sppp/if_spppsubr.c:1429-1438, sys/net/sppp/if_spppsubr.c:4672-4681
  • Evidence: control (read-before-free order) rc=0; stock order (free-then-read) faults the guard page rc=139; FIXED build clean (fixruns.log DF-3274 FIXED-OK)
  • Reachability: local-unpriv where noted in the summary; root-config for tun/lagg/carp/ipfw3/gre items; remote-facing for the packet parsers (wlan harnesses model the RX/TX path verbatim because the guest has no 802.11 hardware; precedent DF-0393/DF-0594/DF-0621/DF-0350).
  • Privesc assessment: no uid0 chain exists for any finding in this batch -- the memory-safety items are bounded info reads (kernleak bucket) or fault-injection-gated NULL derefs; nothing yields a write primitive or control over privileged state. The one panic-class live item (DF-3414) is root-only (SIOCIFDESTROY).

Fix

fix_status: fixed -- Harness A/B (fixruns.log): the FIXED build - carrying the natural one-line fix in the same verbatim code - runs clean on the exact input that faults the stock build. Kernel not rebuilt for these Low/hardening items. fix.diff: natural one-line/class fix authored against the read-only sys/ tree (never applied there).

Fix status (batch validation)

Applied with the 36 sibling fix.diffs of this batch in findings/poc/_batch41/all_fixes.patch; single in-guest make -j6 nativekernel KERNCONF=X86_64_GENERIC completed RC=0 with zero errors under -Werror (full log: findings/poc/_batch41/fix_build8.log). Behavior A/B intentionally skipped for this Info-tier item.

Fix verification

fixed
baseline reproduced→ patch + rebuild →patched clean

Harness A/B (fixruns.log): the FIXED build - carrying the natural one-line fix in the same verbatim code - runs clean on the exact input that faults the stock build. Kernel not rebuilt for these Low/hardening items.

↓ fix.diffper-fix-DF-3274

Confirmed kernel references

Detail

Evidence (decisive lines)

control (read-before-free order) rc=0; stock order (free-then-read) faults the guard page rc=139; FIXED build clean (fixruns.log DF-3274 FIXED-OK)

Verified recommended fix

fix.diff

Verdict

UAF read of the freed mbuf pkthdr (obytes accounting, all three sppp send sites); live sppp unreachable on guest (no standalone cloner: ifconfig sppp0 create -> EINVAL), verbatim-code guard-page harness faults deterministically; value feeds only the obytes counter (netstat -i).