# 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)
- `sys/kern/subr_diskmbr.c:427`
- `sys/kern/subr_diskmbr.c:504`
- `sys/kern/subr_diskmbr.c:509`

## 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