Phase 7: Pwnkit (CVE-2021-4034) detect-only module

First USERSPACE LPE in IAMROOT (every prior module is kernel). Same
iamroot_module interface — the difference is the affected-version
check is package-version-based rather than kernel-version-based.

- modules/pwnkit_cve_2021_4034/:
  - iamroot_modules.{c,h}: detect() locates setuid pkexec (one of
    /usr/bin/pkexec, /usr/sbin/pkexec, /bin/pkexec, /sbin/pkexec,
    /usr/local/bin/pkexec) and parses 'pkexec --version' output.
    Handles BOTH version-string formats: legacy '0.105'/'0.120'
    (older polkit) AND modern bare-integer '121'/'126' (post-0.121
    rename to single-number scheme). Reports VULNERABLE on parse
    failure (conservative).
  - exploit() returns IAMROOT_PRECOND_FAIL with a 'not yet
    implemented' message; full Qualys-PoC follow-up is the next
    commit. ~200 lines including embedded .so generator.
  - MODULE.md documents the bug, affected ranges, distro backport
    landscape (RHEL 7/8, Ubuntu focal/impish, Debian buster/bullseye
    each have their own backported polkit version).
  - Embedded auditd + sigma detection rules:
    auditd: pkexec watch + execve audit
    sigma:  pkexec invocation + suspicious env (GCONV_PATH, CHARSET)

- core/registry.h adds iamroot_register_pwnkit() declaration.
- iamroot.c main() registers pwnkit.
- Makefile gains the pwnkit family as a separate object set.

Verified end-to-end on kctf-mgr (modern polkit 126):
  iamroot --list  → 8 modules
  iamroot --scan  → pwnkit reports 'version 126 ≥ 0.121 (fixed)'
  iamroot --detect-rules --format=auditd | grep pwnkit → emits

modules/pwnkit_cve_2021_4034/MODULE.md

@@ -0,0 +1,57 @@
+# Pwnkit — CVE-2021-4034
+
+> 🔵 **DETECT-ONLY** as of 2026-05-16. Full exploit follows.
+
+## Summary
+
+Polkit's `pkexec` parses argv assuming argc ≥ 1. With `argc == 0`, the
+parsing reads past `argv[0]` into the contiguous envp region, treating
+the first env string as if it were argv[0]. By placing `GCONV_PATH=`
+crafted entries in the environment and naming a controlled file such
+that libc's iconv() loads it as a gconv module, an unprivileged user
+gets code execution as root via the setuid pkexec binary.
+
+Disclosed by Qualys 2022-01-25. Bug existed since pkexec's first
+release in 2009 — affects every distribution shipping a vulnerable
+polkit until 0.121 (or distro backport).
+
+## Affected versions
+
+- **All polkit ≤ 0.120** (i.e., pkexec from 2009 onward) before the
+  fix landed.
+- Patched in upstream **polkit 0.121** (2022-01-25).
+- Distro backports vary:
+  - Ubuntu: 0.105-26ubuntu1.3 (focal), 0.105-31ubuntu0.1 (impish), etc.
+  - Debian: 0.105-31+deb11u1 (bullseye), 0.105-26+deb10u1 (buster)
+  - RHEL: polkit-0.115-13.el7_9 (RHEL 7), polkit-0.117-9.el8_5.1 (RHEL 8)
+
+## IAMROOT detect logic (current)
+
+1. Resolve pkexec binary (`/usr/bin/pkexec` or `which pkexec`)
+2. If not present → IAMROOT_OK (no attack surface)
+3. Run `pkexec --version` and parse version
+4. Compare to known-fixed thresholds; report VULNERABLE if below
+
+## Exploit logic (follow-up)
+
+Canonical Qualys / public Pwnkit PoC:
+
+1. Build a malicious shared object that `exit(setuid(0)); system("/bin/sh")`
+2. Build a `GCONV_PATH=./X` env entry plus `CHARSET=X` so libc's
+   iconv (used by pkexec for argv decoding) loads our .so
+3. `execve("/usr/bin/pkexec", { NULL }, envp)` — argc=0 triggers the
+   read past argv[0], which sees our GCONV_PATH crafted string, then
+   pkexec gives us root context, the gconv module loads our .so as
+   root, we drop to a shell
+
+~200 lines including the embedded .so generator. Phase 7 follow-up
+commit lands the full version.
+
+## Detection rules (shipped)
+
+`detect/auditd.rules` — flags pkexec invocations from non-root.
+
+## References
+
+- https://blog.qualys.com/vulnerabilities-threat-research/2022/01/25/pwnkit-local-privilege-escalation-vulnerability-discovered-in-polkits-pkexec-cve-2021-4034
+- https://nvd.nist.gov/vuln/detail/CVE-2021-4034