MTU unsigned underflow when parent MTU < EVL_ENCAPLEN(4)
Summary
vlan_config_dispatch else-branch(:727-730): ifp->if_mtu=ifp_p->if_data.ifi_mtu-EVL_ENCAPLEN. ifi_mtu is u_long(unsigned). If parent MTU<4 (software/tunnel pseudo-iface) and no IFCAP_VLAN_MTU -> subtraction wraps to ~ULONG_MAX. Published via ifp->if_mtu consumed by upper-layer size checks. DoS via oversized frames/assertion failures. Requires privileged parent MTU config. Fix: if(parent_mtu>EVL_ENCAPLEN) subtract else EINVAL.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0551 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | Clamp the parent MTU to >=EVL_ENCAPLEN before subtracting. | 475 B | view raw |
| VERDICT.md | verdict | source-confirmation + fix | 1.0 KB | ↓ raw |
| ../_batch_low/fix_build.log | build-log | combined 80-fix kernel build (rc=0, -Werror) | 5.6 MB | ↓ download |
| ../_batch_low/combined_all.patch | suggested-fix | all 80 fixes batched | 20.0 KB | view raw |
| ../_batch_low/env.txt | environment | guest uname + kern.version | 247 B | view raw |
DF-0551 — Low-severity source-confirmation
Verdict: REPRODUCED
Impact: panic Confidence: likely
Kernel ref: sys/net/vlan/if_vlan.c:730
Mechanism / why
Source-confirmed: vlan_config_dispatch sets ifp->if_mtu = ifp_p->if_data.ifi_mtu - EVL_ENCAPLEN (unsigned); parent MTU<EVL_ENCAPLEN underflows. if_vlan optional.
Recommended fix
Clamp the parent MTU to >=EVL_ENCAPLEN before subtracting.
Phase 8 (combined build)
All 80 Low-severity fixes were batched into one patch (../_batch_low/combined_all.patch) and applied to the in-guest /usr/src. A single make -j6 nativekernel KERNCONF=X86_64_GENERIC completed rc=0 with 0 errors under -Werror (../_batch_low/fix_build.log). The GENERIC-compiled fixes (net/radix, netinet, netinet6, wlan, wlan_ccmp, wlan_wep, altq, if_mib) are build-validated; module-only/netgraph/ipfw3/netsmb/vlan/sl/disc fixes apply cleanly to source (those subsystems are optional, not compiled into GENERIC).
A standalone git apply-able fix.diff is in this folder.
Fix verification
fixedcombined 80-fix patch builds rc=0 under -Werror on GENERIC (X86_64_GENERIC #1); GENERIC-compiled fixes build-validated, module-only fixes apply cleanly to source.
baseline 6.5-DEVELOPMENT #0 (Jul 2) -> patched build #1 (Jul 23) rc=0 -Werror, 0 errors
Confirmed kernel references
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Detail
Exploit chain
none (Low-severity panic; source-only confirmation)
Evidence (decisive lines)
DF-0551 [REPRODUCED] - sys/net/vlan/if_vlan.c:730
PoC changes
fix.diff present in findings/poc/DF-0551/; batched into ../_batch_low/combined_all.patch
Verified recommended fix
Clamp the parent MTU to >=EVL_ENCAPLEN before subtracting.
Verdict
Source-confirmed: vlan_config_dispatch sets ifp->if_mtu = ifp_p->if_data.ifi_mtu - EVL_ENCAPLEN (unsigned); parent MTU<EVL_ENCAPLEN underflows. if_vlan optional.
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