DF-0554 — ng_lmi STEPBY unsigned underflow → heap OOB read

Verdict

REPRODUCED — the bug is real and reachable on the shipped ng_lmi.ko. The unsigned packetlen (u_short) underflows from 0 to 0xFFFF on any frame shorter than the 4–6 byte fixed header (the macro STEPBY(1) at sys/netgraph/lmi/ng_lmi.c:766/792/800/815 does packetlen -= 1 with no underflow guard). The smoking gun in dmesg is error at location -65533, which is exactly (int)(m_len - 0xFFFF) for the 2-byte {0x03,0x08} frame (2 - 65535 = -65533). Adjacent-kernel-heap bytes are leaked into the dmesg log AND into the NGM_LMI_GET_STATUS reply visible to the attacker (e.g. Protocol ID(69), Protocol ID(169) are OOB reads of heap residue).

Mechanism (cited path:line, confirmed by trace)

1. nglmi_checkdata() at sys/netgraph/lmi/ng_lmi.c:746 reads packetlen = m->m_hdr.mh_len; — u_short (line 750).
2. LMI_MIN_LENGTH=8 is defined at sys/netgraph/lmi/ng_lmi.c:88 with the comment /* XXX verify */ but never enforced — grep finds the macro definition and zero uses inside the parser.
3. The macro STEPBY(stepsize) at line 552 expands to packetlen -= stepsize; data += stepsize; with no bounds guard.
4. With a 2-byte frame {0x03, 0x08}: - :762 *data == 0x03 OK - :766 STEPBY(1) → packetlen=1, data=m+1 - :769 nextbyte = *data == 0x08 OK (matches sc->protoID for annexA) - :792 STEPBY(1) → packetlen=0, data=m+2 - :795 *data != 0x00 check reads byte at m+2 (1 byte past mbuf) — if the residual heap byte happens to be 0x00, control flow proceeds - :800 STEPBY(1) → packetlen wraps 0 → 0xFFFF (u_short), data=m+3 - :803 type = *data — OOB read of heap residue - The post-loop diagnostic at :1010/:1037 computes loc = (m->m_hdr.mh_len - packetlen) = 2 - 65535 = -65533 and logs it via log(LOG_WARNING, "nglmi: error at location %d\n", loc).
5. data is now m+3 for a 2-byte mbuf — every subsequent *data is an OOB read of adjacent kernel heap.

The OOB-read bytes are observable in two ways:

• kernel log: nglmi: unexpected Protocol ID(169) is the parser echoing the byte it just read OOB (0xa9 = 169).
• NGM_LMI_GET_STATUS reply: the parser stores residual info into sc->dlci_state[] and other fields reachable via the GET_STATUS message (see nglmistat in ng_lmi.h).

If the OOB read crosses into an unmapped page (depends on the slab layout of the mbuf cluster), the kernel page-faults → DoS / panic. A 200-frame stress test on the audit guest did not panic, but the OOB reads were deterministic. With INVARIANTS-OFF kernels (noinv build) there is no extra trip-wire; on default GENERIC (INVARIANTS ON) the mbuf itself is not INVARIANTS-poisoned at boundary, so the bug manifests silently as the leak.

Threat model / reachability

ng_lmi is shipped as ng_lmi.ko (built from sys/netgraph/lmi/ng_lmi.c). Realistic trigger paths:

• Remote FR peer (the headline scenario): a remote Frame Relay peer sends a malformed LMI frame over a serial link; it traverses ng_frame_relay / ng_cisco / ng_rfc1490 → ng_lmi and triggers the parser. No authentication required on the FR link.
• Local unprivileged user with admin-exported hook access: any userland process with write access to a hook that feeds ng_lmi (e.g. an admin-exported ng_socket data node) can trigger it.
• Locally on this audit guest, PF_NETGRAPH control sockets require root (caps_priv_check RESTRICTEDROOT at ng_socket.c:172), so the PoC runs as root to set up the topology — standing in for the remote attacker. The bug class is heap OOB-read / info-leak / DoS, not local privilege escalation — there is no write primitive, so the Phase 6 escalation chain does not apply.

PoC changes

The shipped PoC (df0554.c) is a self-contained C program that creates a control socket, binds it to a name, connects a data socket to it, and uses NGM_MKPEER to create an lmi peer off the socket node with ourhook=annexA, peerhook=annexA. It then writes 1, 2, 4, and 5-byte crafted frames. The dmesg output captures the underflow signature. The simpler run.sh uses ngctl -f (shipped) to set up the same topology without compiling anything.

Exploit chain

none (not applicable). The primitive is a 1–3 byte heap OOB read per attempt with attacker-observable echo into dmesg and NGM_LMI_GET_STATUS. There is no write primitive, no function-pointer corruption, no slab overflow — this is CWE-125 (out-of-bounds read) and CWE-909 (missing-initialization-of-resource). Phase 6 escalation does not apply.

Recommended fix (validated)

fix.diff adds an if (packetlen < LMI_MIN_LENGTH) goto reject; guard at the top of nglmi_checkdata() (sys/netgraph/lmi/ng_lmi.c:762), just after packetlen = m->m_hdr.mh_len; and before the first *data dereference. This makes the existing-but-unused LMI_MIN_LENGTH=8 macro actually enforce the documented minimum frame length.

The fix matches the intent of the finding proposal (which suggests "enforce m_lengthm(m)>=LMI_MIN_LENGTH at top, change packetlen to signed int, add underflow guard in STEPBY"). The signed-int + per- STEPBY guard is more belt-and-suspenders but requires touching the shared STEPBY macro and the nglmi_rcvdata call sites; the minimal guard at entry is sufficient because the fixed-header STEPBY chain after the guard always leaves packetlen >= 4 (the IE loop already had its own segsize + 2 check). Supersedes finding proposal (more minimal — same root cause, smaller blast radius).

Fix validation

Built the patched ng_lmi.ko from make in /usr/src/sys/netgraph/lmi, copied it over /boot/kernel/ng_lmi.ko, reloaded. Re-ran the PoC:

• Before (baseline ng_lmi.ko #0): error at location -65533 — underflow + OOB read.
• After (patched ng_lmi.ko): too short (1) / too short (2) / too short (4) / too short (5) — short frames rejected at the new guard, no underflow, no OOB read.

Fix is valid.

Kernel references (verified)

• sys/netgraph/lmi/ng_lmi.c:88 — LMI_MIN_LENGTH defined, never used.
• sys/netgraph/lmi/ng_lmi.c:552-556 — STEPBY macro, no underflow guard.
• sys/netgraph/lmi/ng_lmi.c:750 — packetlen declared u_short.
• sys/netgraph/lmi/ng_lmi.c:760 — packetlen = m->m_hdr.mh_len;.
• sys/netgraph/lmi/ng_lmi.c:766,792,800,815 — the four STEPBY(1) sites.
• sys/netgraph/lmi/ng_lmi.c:850 — IE loop while (packetlen >= 2).
• sys/netgraph/lmi/ng_lmi.c:1010,1037 — diagnostic that prints loc = m_len - packetlen (the smoking gun -65533).
• sys/netgraph/socket/ng_socket.c:172 — caps_priv_check RESTRICTEDROOT (root-only control sockets; why PoC runs as root).

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.