⬢ DragonFlyBSD Kernel Audit
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DF-0310

Non-atomic increment of global fragment ID (ip6_id): data race / predictable fragment IDs

Summary

ip6_id++(:753) non-atomic RMW no lock/crit/atomic. Multiple netisr CPUs race -> lost increments, duplicate IDs for unrelated flows, more predictable sequence. Weakens fragment ID as authenticator against off-path IPv6 fragment injection.

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PoC verification

Evidence pack

findings/poc/DF-0310 · 5 files
FileTypeDescriptionSize
fix.diff suggested-fix git-apply-able fix for DF-0310 346 B view raw
VERDICT.md verdict source-confirmation + fix summary 590 B ↓ raw
build.sh build-log combined-build validation note 333 B view raw
run.sh run-log source-only (no runtime PoC) 133 B view raw
env.txt environment guest uname + build env 218 B view raw
VERDICT.md verdict source-confirmation + fix summary
↓ download raw

DF-0310 -- Low

Verdict: REPRODUCED (source-confirmed)

REAL: ip6_id++ (line 753) is a non-atomic RMW with no lock/crit; concurrent netisr CPUs race -> lost increments, duplicate IDs, more predictable sequence (weakens fragment ID as an off-path-injection authenticator).

Impact ceiling

none

Cited refs

Fix

Use atomic_fetchadd_int(&ip6_id, 1) for the increment.

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/netinet6/ip6_output.c:753 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/netinet6/ip6_output.c:753; combined-fix kernel 'NK_DONE rc=0' (0 errors,-Werror).

PoC changes

authored fix.diff (findings/poc/DF-0310/fix.diff); validated in the combined 60-finding kernel build.

Verified recommended fix

Use atomic_fetchadd_int(&ip6_id, 1) for the increment. Full diff: findings/poc/DF-0310/fix.diff.

Verdict

REAL: ip6_id++ (line 753) is a non-atomic RMW with no lock/crit; concurrent netisr CPUs race -> lost increments, duplicate IDs, more predictable sequence (weakens fragment ID as an off-path-injection authenticator).