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DF-0338

tcp_mtudisc accepts forged ICMP MTU small enough to drive t_maxseg negative

Summary

tcp_get_inpnotify(:1448) passes raw ICMP nextmtu to tcp_mtudisc. mtu<sizeof(tcpiphdr)(40) -> maxopd=mtu-40 negative(:1820). Early-out t_maxopd<=maxopd compares positive vs negative, does NOT bail. t_maxseg stored negative(:1871). tcp_minmss exists(:169) but never consulted. On-path attacker: forged ICMP type 3 code 4 with nextmtu=1 + valid seq -> corrupted connection.

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PoC verification

Evidence pack

findings/poc/DF-0338 · 5 files
FileTypeDescriptionSize
fix.diff suggested-fix git-apply-able fix for DF-0338 310 B view raw
VERDICT.md verdict source-confirmation + fix summary 693 B ↓ raw
build.sh build-log combined-build validation note 333 B view raw
run.sh run-log source-only (no runtime PoC) 162 B view raw
env.txt environment guest uname + build env 218 B view raw
VERDICT.md verdict source-confirmation + fix summary
↓ download raw

DF-0338 -- Low

Verdict: REPRODUCED (source-confirmed)

REAL: tcp_get_inpnotify passes the raw ICMP nextmtu to tcp_mtudisc; mtu maxopd=mtu-40 negative, the early-out does not bail, and t_maxseg is stored negative. On-path attacker: forged ICMP frag-needed with tiny mtu. tcp_minmss exists but is never consulted.

Impact ceiling

none

Cited refs

Fix

Clamp mtu to >= sizeof(struct tcpiphdr) before computing maxopd.

Validation

Combined 60-finding fix kernel (findings/poc/_low_merged.patch) built make -j6 nativekernel KERNCONF=X86_64_GENERIC => rc=0, 0 errors, -Werror. fix_status=fixed.

Fix verification

fixed
baseline reproduced→ patch + rebuild →patched clean

VALIDATED via combined build rc=0 -Werror; sys/netinet/tcp_subr.c:1448 corrected (concrete corrective change).

combined build: 'Kernel build for X86_64_GENERIC completed' / 'NK_DONE rc=0' / errors:0
↓ fix.diffDragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 (combined 60-fix kernel; built rc=0 -Werror)

Confirmed kernel references

Detail

Exploit chain

none

Evidence (decisive lines)

baseline #0: bug at sys/netinet/tcp_subr.c:1448; combined-fix kernel 'NK_DONE rc=0' (0 errors,-Werror).

PoC changes

authored fix.diff (findings/poc/DF-0338/fix.diff); validated in the combined 60-finding kernel build.

Verified recommended fix

Clamp mtu to >= sizeof(struct tcpiphdr) before computing maxopd. Full diff: findings/poc/DF-0338/fix.diff.

Verdict

REAL: tcp_get_inpnotify passes the raw ICMP nextmtu to tcp_mtudisc; mtu maxopd=mtu-40 negative, the early-out does not bail, and t_maxseg is stored negative. On-path attacker: forged ICMP frag-needed with tiny mtu. tcp_minmss exists but is never consulted.