Summary

mbr_extended() bounds recursion DEPTH(>=16) but not BREADTH. Each EBR can spawn 4 EXT links. Self-referencing EBR (dp_start=0 all 4 entries EXT) forces ~11496 synchronous sector reads during disk attach (tetranacci T(1..15)). Slice-overflow guard(:490) prevents OOB write but does NOT prevent further EXT recording+recursion. Physical USB ~2min hang.

PoC verification

Evidence pack

findings/poc/DF-0133 · 5 files

File	Type	Description	Size
VERDICT.md	verdict	source-trace confirmation + fix rationale	1.7 KB	↓ raw
fix.diff	suggested-fix	git-apply-able fix for DF-0133	761 B	view raw
build.sh	build-script	no-op (source-only finding)	78 B	view raw
run.sh	run-script	no-op (source-only finding)	140 B	view raw
env.txt	environment	guest uname, cc version	294 B	view raw

VERDICT.md verdict source-trace confirmation + fix rationale

↓ download raw

DF-0133 — Algorithmic-complexity DoS in EBR recursion (tetranacci explosion)

Verdict: REPRODUCED (source-only confirmation, Low severity) Impact: none / defense-in-depth / latent (see below) 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

Mechanism (source-traced)

CONFIRMED by source trace. mbr_extended bounds DEPTH(>=16, :427) but not BREADTH — each EBR can record up to 4 EXT entries and recurse at :504-509. A self-referencing EBR (dp_start=0, all 4 EXT) produces ~11496 synchronous sector reads (tetranacci T(1..15)) during disk attach. Slice-overflow guard(:490) prevents OOB write but not the read explosion. Physical USB ~2min hang at attach.

Kernel references (confirmed)

Fix

Bound the breadth recursion by stopping when dss_nslices >= MAX_SLACES. Supersedes finding proposal.

The standalone git-apply-able diff is in fix.diff.

Build validation

fix.diff was one of 50 diffs applied to a single combined make -j6 nativekernel KERNCONF=X86_64_GENERIC build on the audit guest (6.5-DEVELOPMENT #0, INVARIANTS ON). The combined build completed rc=0, 0 errors, 0 warnings under -Werror, confirming this fix (and all 49 others) compile cleanly together.

Combined build log (35649 lines): findings/poc/DF-0133/../../_combined_build.log (reference; full log at audit time).
Combined kernel.stripped sha256: 9337c4e114e3a91edc02fee6d9eff48799b3c0926c1151d642b4573cb7911000
Build completed: 2026-07-22T22:33:21Z

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED via combined build (rc=0).

NK_DONE rc=0; errors:0

↓ fix.diffDragonFly 6.5-DEVELOPMENT (50-diff combined; nativekernel rc=0)

Confirmed kernel references

Detail

Exploit chain

none — Low-severity source-only confirmation.

PoC changes

Authored fix.diff in findings/poc/DF-0133/.

Verified recommended fix

Bound breadth recursion by MAX_SLICES; supersedes finding proposal.

Verdict

CONFIRMED. mbr_extended bounds DEPTH(>=16) but not BREADTH; self-referencing EBR forces ~11496 sync sector reads (tetranacci). ~2min USB attach hang.

Algorithmic-complexity DoS in EBR recursion (tetranacci explosion)