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

Ova stranica dokumentuje TOCTOU race condition u Linux/Android POSIX CPU timerima koji može korumpirati stanje timera i srušiti kernel, a u određenim okolnostima može se iskoristiti za eskalaciju privilegija.

Pogođena komponenta: kernel/time/posix-cpu-timers.c
Primitiv: expiry vs deletion race under task exit
Osetljivo na konfiguraciju: CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (IRQ-context expiry path)

Kratak pregled unutrašnjih mehanizama (relevantno za eksploataciju)

Tri CPU clock-a vode obračun za timere preko cpu_clock_sample():
CPUCLOCK_PROF: utime + stime
CPUCLOCK_VIRT: samo utime
CPUCLOCK_SCHED: task_sched_runtime()
Kreiranje timera povezuje timer sa task/pid-om i inicijalizuje timerqueue čvorove:

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 umeće u per-base timerqueue i može ažurirati keš sledećeg isteka:

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;
}

Brza putanja izbegava skupe operacije osim ako keširani zapisi o isteku ne ukazuju na moguće okidanje:

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;
}

Isticanje prikuplja istekle tajmere, označava ih kao okinute, sklanja ih sa reda; stvarna isporuka se odlaže:

#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;
}

Dva režima obrade isteka

CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y: istek se odlaže putem task_work na ciljanom tasku
CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n: istek se obrađuje direktno u IRQ kontekstu

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

Na IRQ-context putanji, firing list se obrađuje izvan 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

update_process_times() triggers run_posix_cpu_timers() in IRQ context for the exiting task.
collect_timerqueue() sets ctmr->firing = 1 and moves the timer to the temporary firing list.
handle_posix_cpu_timers() drops sighand via unlock_task_sighand() to deliver timers outside the lock.
Immediately after unlock, the exiting task can be reaped; a sibling thread executes posix_cpu_timer_del().
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;

Ovo sprečava ulazak u handle_posix_cpu_timers() za zadatke koji su u procesu izlaska, eliminišući prozor u kome posix_cpu_timer_del() može da propusti it.cpu.firing i da se trka sa obradom isteka.

Uticaj

Oštećenje kernel memorije u strukturama timera tokom istovremenog isteka/brisanja može dovesti do trenutnih padova (DoS) i predstavlja snažan primitiv za eskalaciju privilegija zbog mogućnosti proizvoljne manipulacije stanjem kernela.

Pokretanje buga (bezbedni, ponovljivi uslovi) Build/config

Obezbedite CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n i koristite kernel bez fix-a za exit_state gating.

Strategija izvršavanja (runtime)

Ciljajte nit koja je na izlasku i pridružite joj CPU timer (po niti ili za ceo proces):
For per-thread: timer_create(CLOCK_THREAD_CPUTIME_ID, ...)
For process-wide: timer_create(CLOCK_PROCESS_CPUTIME_ID, ...)
Podesite veoma kratko početno isteknuće i mali interval kako biste maksimalizovali ulaze u IRQ-putanju:

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");
}

Iz srodnog thread-a, istovremeno obrišite isti timer dok ciljni thread izlazi:

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

Pojačivači trke: visok scheduler tick rate, CPU opterećenje, ponovljeni ciklusi izlaska/ponovnog kreiranja thread-ova. Crash se obično manifestuje kada posix_cpu_timer_del() preskoči uočavanje firing-a zbog neuspeha pri task lookup/locking odmah nakon unlock_task_sighand().

Detection and hardening

Mitigation: primeniti exit_state guard; po mogućstvu omogućiti CONFIG_POSIX_CPU_TIMERS_TASK_WORK.
Observability: dodati tracepoints/WARN_ONCE oko unlock_task_sighand()/posix_cpu_timer_del(); alarmirati kada se it.cpu.firing==1 uoči zajedno sa neuspehom cpu_timer_task_rcu()/lock_task_sighand(); pratiti neusaglašenosti u timerqueue oko izlaska task-a.

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

Objavljeno ponašanje predstavlja pouzdan kernel crash primitive; da bi se to pretvorilo u privilege escalation obično je potreban dodatni kontrolisani overlap (object lifetime ili write-what-where uticaj) izvan opsega ovog sažetka. Smatrajte svaki PoC potencijalno destabilizujućim i pokrećite ga samo u emulatorima/VMs.

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

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