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The shipped module used a bogus chown-the-merged-view technique that never
worked. Rewrote it as a faithful port of the public PoC (xkaneiki): a libfuse
filesystem exports a setuid-root /file (st_uid=0, mode 04777), mounted in the
init ns via the setuid fusermount helper; then unshare(USER|NS) + overlay with
that FUSE mount as lowerdir; open(merged/file, O_WRONLY) triggers copy-up which
materialises upper/file as a genuine setuid-root binary; the unprivileged parent
execs it for real root.
VM-verified landing real root on Ubuntu 22.04.0 / 5.15.0-25 (uid=0 witnessed
out-of-band: /tmp/skeletonkey-ovlsu-pwned shows uid=0(root), and the dropped
setuid /tmp/.suid_bash runs with euid=0).
Four things were each required and took isolation to find:
1. Overlay refuses a userns-mounted FUSE lowerdir (ENOSYS) -> FUSE must be
mounted in the init ns via fusermount (libfuse). A raw /dev/fuse server was
tried and abandoned (fragile; destabilised the kernel on malformed INIT).
2. fuse2 low-level API (fuse_mount/fuse_new/fuse_loop_mt, empty args); fuse_main
advertises copy_file_range caps that ENOSYS at copy-up with no fallback.
3. read_buf callback (copy-up splice read path).
4. ioctl callback (copy-up's FS_IOC_GETFLAGS) — the last missing piece.
libfuse is linked conditionally via pkg-config (fuse/fuse3), matching the
pack2theroot+libglib precedent; without it the module stubs to PRECOND_FAIL.
Makefile detects it and adds $(OSU_LIBS) to the link. Module header + opsec
notes rewritten to the real technique; docs/EXPLOITED.md updated. 148-test unit
harness green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0118iUgHY44hdRtANgyCmu7y
118 lines
8.7 KiB
Markdown
118 lines
8.7 KiB
Markdown
# Exploit verification ledger
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**What this is:** results of actually *running each exploit* against a genuinely
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vulnerable VM and confirming `uid=0` **out of band** (an independent root-owned
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write / `/etc/shadow` read / setuid-bash sentinel — never the module's own
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self-report). This is distinct from `docs/VERIFICATIONS.jsonl`'s historical
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records, which only checked that `detect()` returns the right verdict.
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Harness: rootless qemu/KVM over frozen point-release cloud images (unpatched →
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vulnerable by default), driver in the session scratch dir. Root witnessed via
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`witness.sh` (shell-probe + setuid-bash finisher + module sentinels + `/etc/passwd`
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tamper check).
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> **Headline finding:** the corpus was only ever *detect*-verified, never
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> *exploit*-verified. Running the exploits shows a mix of genuinely-working,
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> honestly-failing, and **falsely-succeeding** modules. Two flagship modules
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> reported `EXPLOIT_OK` while obtaining **no root at all** (`pwnkit`,
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> `ptrace_traceme`) — a false positive from the dispatcher's "execve
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> transferred → clean child exit = OK" path. Both are addressed below.
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## Confirmed landing root (uid=0 witnessed out of band)
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| module | CVE | target | notes |
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|---|---|---|---|
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| `refluxfs` | CVE-2026-64600 | Rocky 9.8 / 5.14.0-687.el9 | full chain, `/etc/passwd` → root (earlier) |
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| `overlayfs` | CVE-2021-3493 | Ubuntu 20.04.0 / 5.4.0-26 | userns + xattr copy-up, clean |
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| `overlayfs_setuid` | CVE-2023-0386 | Ubuntu 22.04.0 / 5.15.0-25 | **after a full rewrite** — see below |
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| `pwnkit` | CVE-2021-4034 | Ubuntu 20.04.0 / polkit 0.105-26ubuntu1 | **after a fix** — see below |
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| `sudo_runas_neg1` | CVE-2019-14287 | Ubuntu 18.04.2 / sudo 1.8.21p2 + sudoers `(ALL,!root)` | `sudo -u#-1` → uid 0 |
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## Fixed this session
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- **`pwnkit`** — reported `EXPLOIT_OK` but did **not** root (glibc "Could not
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open converter … to PWNKIT"). Root cause: missing the `GCONV_PATH=.`
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re-injection directory + `chdir(workdir)`. Fixed → now lands real root on a
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vulnerable host. (commit `24b839e`)
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- **`ptrace_traceme`** — reported `EXPLOIT_OK` but did **not** root; the bundled
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ptrace sequence is a non-working placeholder (it `PTRACE_ATTACH`es to the
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tracer, which by then has reparented to init → `EPERM`). Made **honest**
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(verifies `euid==0`, otherwise `EXPLOIT_FAIL`); a faithful CVE-2019-13272 port
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(Jann Horn / bcoles: `PTRACE_TRACEME` + setuid execve + registered polkit
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agent) is still required to land root. (commit `59cc2be`)
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- **`overlayfs_setuid`** (CVE-2023-0386) — **rewritten and now lands real root**
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(uid=0 witnessed out-of-band on Ubuntu 22.04.0 / 5.15.0-25). The shipped
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module used a bogus `chown`-the-merged-view technique that never worked. The
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real bug needs a **FUSE lower layer** exporting a setuid-root file; overlay
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copy-up then materialises it in the real upper as a genuine setuid-root
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binary. Key findings from the port (all four were required):
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1. Overlay refuses a **userns-mounted** FUSE lowerdir (ENOSYS) — FUSE must
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be mounted in the **init ns** via the setuid `fusermount` helper (libfuse
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does this). A raw `/dev/fuse` server was tried and abandoned: its INIT
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handshake needs `poll()` on the non-blocking fd, and a malformed reply
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destabilised the kernel — fragile and inappropriate. libfuse is linked
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conditionally (pkg-config `fuse`/`fuse3`), matching `pack2theroot`.
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2. **fuse2** low-level API (`fuse_mount`/`fuse_new`/`fuse_loop_mt`, empty
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args) — `fuse_main` advertises splice/copy_file_range caps that make the
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kernel attempt `copy_file_range` at copy-up → ENOSYS with no fallback.
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3. **`read_buf`** callback (copy-up's splice read path).
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4. **`ioctl`** callback — copy-up issues `FS_IOC_GETFLAGS` on the lower; a
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server without an ioctl handler returns ENOSYS and copy-up fails. This
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was the last missing piece.
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Debugging was isolated by driving the exploit orchestration against the public
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PoC's `./fuse`, then swapping servers, then comparing `fops`.
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- **`cgroup_release_agent`** — two real bugs fixed (commit `8c45b2b`): it read
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`getuid()` **after** `unshare(CLONE_NEWUSER)` (→ `65534`, so `uid_map` write
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was `"0 65534 1"` → EPERM), and it omitted `CLONE_NEWCGROUP` (→ cgroup-v1
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mount EPERM). Now the userns+cgroupns+mount setup is correct. It still can't
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root a **bare** unprivileged user on a stock systemd host: every v1 controller
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is pre-mounted (its `release_agent` is init-owned → EACCES from the userns)
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and a fresh named hierarchy is refused. Reachable in a **container** context
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(CAP_SYS_ADMIN / an ownable cgroup) — matches its "host root from rootless
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container" framing. The `getuid()`-after-`unshare` bug is a pattern to grep
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for across the other userns modules.
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## Needs a faithful PoC port (genuinely vulnerable target, exploit doesn't land)
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| module | CVE | target tested | what's wrong |
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|---|---|---|---|
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| `ptrace_traceme` | CVE-2019-13272 | Ubuntu 18.04.2 / 4.15.0-50 | placeholder technique (now honest); needs the real cred-transfer PoC (polkit agent) — substantial port |
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| `overlayfs_setuid` | CVE-2023-0386 | Ubuntu 22.04.0 / 5.15.0-25.25 | **Kernel confirmed vulnerable empirically** — the upstream PoC (xkaneiki, libfuse) pops root here (`uid=0(root)`, root-owned witness). The working technique: mount a FUSE fs exporting `/file` (st_uid=0, mode 04777) in the **init ns** via the setuid `fusermount3` helper, then overlay-in-userns with that FUSE lowerdir + copy-up. My module's non-FUSE `chown` variant yields `upper/file` uid=1000 (no escalation); mounting FUSE **inside** the userns → overlay `ENOSYS`. Attempted a self-contained **raw `/dev/fuse`** port: got the `fusermount` fd-passing handshake (`SCM_RIGHTS`) + mount working, but the server hits `EINVAL` on `read()` after `FUSE_INIT` (non-blocking fd → needs `poll()`), and even with poll/buffer fixes the raw server serving was flaky and repeatedly **wedged/rebooted the VM** — i.e. the raw protocol reimplementation is fragile and can destabilise the target, which is *worse* for the corpus than a lib dependency. **Conclusion: use libfuse** (proven, robust; matches the `pack2theroot` conditional-lib precedent). Port is scoped and ready; needs a clean session to implement + verify. |
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| `sudoedit_editor` | CVE-2023-22809 | Ubuntu 22.04.0 / sudo 1.9.9 | vulnerable sudo version + sudoers `sudoedit` grant present; module returns `EXPLOIT_FAIL` — the `SUDO_EDITOR`/`--` arg-injection reaches sudoedit's writable-dir guard. Needs a target-file whose dir the injection can satisfy (e.g. `/etc/passwd`), and module debugging. Structural, tractable next. |
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| `sudo_samedit` | CVE-2021-3156 | Ubuntu 18.04.2 + 20.04.0 | honest `EXPLOIT_FAIL`; Baron Samedit is heap/libc-version-specific, needs a per-target tuned PoC. Hardest. |
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## Inconclusive (detect version-blind vs vendor backport)
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| module | CVE | note |
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|---|---|---|
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| `dirty_pipe` | CVE-2022-0847 | Ubuntu 22.04.0's 5.15.0-25.25 almost certainly carries the backported fix (USN-5317); `detect()` is version-only so says VULNERABLE. Needs a genuinely pre-fix kernel (e.g. 5.13.0-19, or mainline 5.16.10) to verify the exploit. |
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## Not yet exploit-tested (the bulk — ~30 modules)
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- **🟡 kernel primitives** (`nf_tables`, `nft_set_uaf`, `nft_payload`,
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`nft_fwd_dup`, `netfilter_xtcompat`, `af_packet`, `af_packet2`, `af_unix_gc`,
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`cls_route4`, `fuse_legacy`, `stackrot`, `sequoia`, `nft_pipapo`, `vsock_uaf`,
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`pintheft`): return `EXPLOIT_FAIL` by design; landing root needs a per-kernel
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`--dump-offsets` + `--full-chain` (modprobe_path finisher) pass on a vulnerable
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kernel.
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- **Structural userspace** (`sudoedit_editor`, `sudo_chwoot`, `sudo_host`):
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need specific sudo versions + sudoers config; likely tractable.
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- **2026 CVEs** (`copy_fail` ×5, `dirtydecrypt`, `fragnesia`, `cifswitch`,
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`nft_catchall`, `ptrace_pidfd`): need vulnerable 2026 kernels; the reconstructed
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race triggers (`bad_epoll`, `ghostlock`, `nft_catchall`) are deliberately
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under-driven and won't pop root by design.
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- **Environment-blocked**: `vmwgfx` (VMware guest only), `dirty_cow` (needs ≤4.4),
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`mutagen_astronomy` (CentOS 6 / Debian 7).
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- **D-Bus/desktop** (`pack2theroot`, `udisks_libblockdev`): need the polkit/D-Bus
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stack + a provisioner rule.
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## Method notes for continuation
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- Frozen images: `cloud-images-archive.ubuntu.com/releases/<name>/release-<date>/`
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are unpatched and vulnerable-by-default for CVEs disclosed after that date — far
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easier than downgrading packages on current images.
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- gcc must be present *in* the VM (several exploits compile payloads at runtime);
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on EOL LTS, point apt at the archive main pocket.
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- **Always verify root out of band.** The module self-report is not trustworthy
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(two flagships lied). `witness.sh` is the reference check.
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