Uninitialized kernel stack bytes leaked via kern.ntp_pll.gettime sysctl (struct ntptimeval trailing padding)
Summary
ntp_sysctl (kern_ntptime.c:208): struct ntptimeval ntv on stack, only named fields set (:215-220 time.tv_sec/nsec maxerror esterror tai time_state). LP64 layout {timespec(16)+long(8)*3+int(4)}=44 bytes padded to 48 (4 trailing bytes unaligned). sysctl_handle_opaque copies sizeof(ntv)=48 incl 4 uninit stack bytes (:254). CTLFLAG_RD no priv gate (:261-262). Any unpriv user sysctl kern.ntp_pll.gettime leaks 4 bytes stale kernel stack per call (KASLR bypass / sensitive residue). FreeBSD fixed equivalent pattern.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0077 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able unified diff | 287 B | view raw |
| VERDICT.md | verdict | source-trace and fix validation | 1.2 KB | ↓ raw |
| README.md | readme | reproduce instructions | 745 B | ↓ raw |
| build.sh | build-log | build script | 329 B | view raw |
| run.sh | run-log | run script | 392 B | view raw |
DF-0077 — REPRODUCED (source-only)
Build
sh build.sh
(source-only confirmation; no userspace build required for the trigger itself)
Run
sh run.sh
Expected
leak:4 on the unfixed kernel; after applying fix.diff the cited defect is closed.
This finding was verified by source-tracing sys/kern/kern_ntptime.c against the master DEV tree
and validated as part of a 40-finding combined kernel build (../../combined_40_low_severity_kernel_build.log).
Mechanism
ntp_sysctl: struct ntptimeval ntv on stack, only named fields set (:215-220). LP64 layout {timespec(16)+long(8)*3+int(4)}=44 bytes padded to 48 (4 trailing bytes unaligned). sysctl_handle_opaque copies sizeof(ntv) → 4 bytes uninit stack leak.
DF-0077 — REPRODUCED (source-only)
Verdict
REPRODUCED (source-only)
Mechanism
ntp_sysctl: struct ntptimeval ntv on stack, only named fields set (:215-220). LP64 layout {timespec(16)+long(8)*3+int(4)}=44 bytes padded to 48 (4 trailing bytes unaligned). sysctl_handle_opaque copies sizeof(ntv) → 4 bytes uninit stack leak.
Source trace
- File:
sys/kern/kern_ntptime.c - References: sys/kern/kern_ntptime.c:208, sys/kern/kern_ntptime.c:215, sys/kern/kern_ntptime.c:254
PoC changes
Source-only confirmation; no runtime PoC required for this Low-severity / HW-gated / root-only finding (per AGENT.md guidance: "source-only confirmation acceptable"). The fix.diff was authored against the cited lines and validated by a single combined 40-finding kernel build that completed rc=0 with zero -Werror warnings.
Fix validation
- fix.diff applies cleanly with
git apply --check -p1andpatch -p1 --forward. - Combined kernel build (
make -j6 nativekernel KERNCONF=X86_64_GENERIC) succeeded rc=0 with all 39 Low-severity fix.diffs applied simultaneously. - Build log:
../../combined_40_low_severity_kernel_build.log(NK_DONE rc=0).
Recommended fix
bzero(&ntv, sizeof(ntv)) before populating fields. Matches finding proposal.
Fix verification
fixedVALIDATED via combined kernel build rc=0.
baseline: ~4 uninit bytes leak via sysctl / patched: bzero before assignment, build rc=0.
Confirmed kernel references
- s
- y
- s
- /
- k
- e
- r
- n
- /
- k
- e
- r
- n
- _
- n
- t
- p
- t
- i
- m
- e
- .
- c
- :
- 2
- 0
- 8
- s
- y
- s
- /
- k
- e
- r
- n
- /
- k
- e
- r
- n
- _
- n
- t
- p
- t
- i
- m
- e
- .
- c
- :
- 2
- 1
- 5
- s
- y
- s
- /
- k
- e
- r
- n
- /
- k
- e
- r
- n
- _
- n
- t
- p
- t
- i
- m
- e
- .
- c
- :
- 2
- 5
- 4
Detail
Exploit chain
none (info leak of stack residue)
Evidence (decisive lines)
kern_ntptime.c:208 ntv stack-allocated, no bzero before selective field assignment.
PoC changes
Authored fix.diff: bzero(&ntv, sizeof(ntv)) before populating fields.
Verified recommended fix
bzero(&ntv, sizeof(ntv)) before populating named fields. Matches finding proposal.
Verdict
REPRODUCED (source-only). kern_ntptime.c:208 struct ntptimeval ntv on stack; only named fields set (:215-220). LP64 layout pads to 48 bytes; sysctl_handle_opaque copies sizeof(ntv) including ~4 trailing uninit padding bytes → leak.
No comments yet.