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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nposix-cpu-timers: Prevent UAF caused by non-leader exec() race\n\nWongi and Jungwoo decoded and reported a non-leader exec() related race\nwhich can result in an UAF:\n\n sys_timer_delete()\t\t\texec()\n   posix_cpu_timer_del()\n   // Observes old leader\n   p = pid_task(pid, pid_type);\t\tde_thread()\n   \t\t\t\t\t  switch_leader();\n\t\t\t\t\t  release_task(old_leader)\n\t\t\t\t\t    __exit_signal(old_leader)\n\t\t\t\t\t      sighand = lock(old_leader, sighand);\n\t\t\t\t\t      posix_cpu_timers*_exit();\n   sighand = lock_task_sighand(p)\t      unhash_task(old_leader);\n     sh = lock(p, sighand)\t    \t      old_leader-\u003esighand = NULL;\n\t\t\t\t\t      unlock(sighand);\n     (p-\u003esighand == NULL)\n\tunlock(sh)\n\treturn NULL;\n\n   // Returns without action\n   if(!sighand)\n      return 0;\n   free_posix_timer();\n\nThis is \"harmless\" unless the deleted timer was armed and enqueued in\np-\u003esignal because on exec() a TGID targeted timer is inherited.\n\nAs sys_timer_delete() freed the underlying posix timer object\nrun_posix_cpu_timers() or any timerqueue related add/delete operations on\nother timers will access the freed object\u0027s timerqueue node, which results\nin an UAF.\n\nThere is a similar problem vs. posix_cpu_timer_set(). For regular posix\ntimers it just transiently returns -ESRCH to user space, but for the use\ncase in do_cpu_nanosleep() it\u0027s the same UAF just that the k_itimer is\nallocated on the stack.\n\nAlso posix_cpu_timer_rearm() fails to rearm the timer, which means it stops\nto expire.\n\nWhile debating solutions Frederic pointed out another problem:\n\n   posix_cpu_timer_del(tmr)\n\t\t\t\t\t__exit_signal(p)\n\t\t\t\t\t  posix_cpu_timers*_exit(p);\n\t\t\t\t\t  unhash_task(p);\n\t\t\t\t\t  p-\u003esighand = NULL;\n     sh = lock_task_sighand(p)\n        sighand = p-\u003esighand;\n\tif (!sighand)\n\t    return NULL;\n\tlock(sighand);\n\n     if (!sh)\n\tWARN_ON_ONCE(timer_queued(tmr));\n\nOn weakly ordered architectures it is not guaranteed that\nposix_cpu_timer_del() will observe the stores in posix_cpu_timers*_exit()\nwhen p-\u003esighand is observed as NULL, which means the WARN() can be a false\npositive.\n\nSolve these issues by:\n\n  1) Changing the store in __exit_signal() to smp_store_release().\n\n  2) Adding a smp_acquire__after_ctrl_dep() into the !sighand path\n     of lock_task_sighand().\n\n  3) Creating a helper function for looking up the task and locking sighand\n     which does not return when sighand == NULL. Instead it retries the\n     task lookup and only if that fails it gives up.\n\n  4) Using that helper in the three affected functions.\n\n#1/#2 ensures that the reader side which observes sighand == NULL also\nobserves all preceeding stores, i.e. the stores in posix_cpu_timers*_exit()\nand the ones in unhash_task().\n\n#3 ensures that the above described non-leader exec() situation is handled\ngracefully. When the task lookup returns the old leader, but sighand ==\nNULL then it retries. In the non-leader exec() case the subsequent task\nlookup will observe the new leader due to #1/#2. In normal exit() scenarios\nthe subsequent lookup fails.\n\nWhen the task lookup fails, the function also checks whether the timer is\nstill enqueued and issues a warning if that\u0027s the case. Unfortunately there\nis nothing which can be done about it, but as the task is already not\nlonger visible the timer should not be accessed anymore. This check also\nrequires memory ordering, which is not provided when the first lookup\nfails. To achieve that the check is preceeded by a smp_rmb() which pairs\nwith the smp_wmb() in write_seqlock() in __exit_signal(). That ensures that\nthe stores in posix_cpu_timers*_exit() are visible.\n\nThe history of the non-leader exec() issue goes back to the early days of\nposix CPU timers, which stored a pointer to the group leader task in the\ntimer. That obviously fails when a non-leader exec() switches the leader.\ncommit e0a70217107e (\"posix-cpu-timers: workaround to suppress the problems\nwith mt exec\") added a temporary workaround for that in 2010 which surv\n---truncated---"
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The vulnerability is reached purely through local syscalls (`timer_create`/`timer_settime`/`timer_delete`/`clock_nanosleep`) raced against `execve()` from a non-leader thread in the attacker\u0027s own process. No network or remote data path reaches posix-cpu-timers.\nAC:L - The attacker controls both sides of the race \u2014 one thread arms and deletes a process-wide CPU timer while a non-leader thread in the same process calls `execve()` \u2014 and can loop indefinitely with CPU pinning/thread spraying to widen the window between `pid_task()` and `lock_task_sighand()`. No condition depends on anything outside attacker control.\nPR:L - Any ordinary unprivileged local user can create and delete `CLOCK_PROCESS_CPUTIME_ID` timers; `pid_for_clock()` performs no capability check when the encoded PID is 0, and `CONFIG_POSIX_TIMERS` is `default y` on essentially all deployments.\nUI:N - The attacker performs every step \u2014 timer creation, arming, deletion, and the concurrent non-leader exec \u2014 entirely within its own multithreaded process. No victim action is involved.\nS:U - The corruption stays within the kernel\u0027s own security authority; it is a standard local kernel memory-corruption/privilege-escalation primitive with no VM, IOMMU, or hypervisor boundary crossed.\nC:H - The stale timerqueue node yields UAF reads of the freed `k_itimer` and causes `current` (a live `task_struct` pointer) plus rbtree pointers to be written into the freed/reallocated slot, and the freed object can be reclaimed by another attacker-created `k_itimer`, enabling kernel pointer disclosure and broad memory read.\nI:H - `collect_timerqueue()` writes into freed memory (`ctmr-\u003efiring`, `ctmr-\u003ehandling`, rbtree rebalancing, list insertion) and the `do_cpu_nanosleep()` variant does so into a live, attacker-groomed kernel stack frame, followed by `cpu_timer_fire()` dereferencing attacker-controlled `it_process`/`it_signal` \u2014 a control-flow-hijack-capable write primitive.\nA:H - Even without exploitation the dangling timerqueue node produces rbtree/list corruption and a kernel oops, and the pre-fix `WARN_ON_ONCE(timerqueue_node_queued(...))` is an immediate panic under `panic_on_warn`; the race can be triggered repeatedly by any local user."
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          "url": "https://git.kernel.org/stable/c/67aa823e3e8c229c6d374df79c804f6721cb83b6"
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          "url": "https://git.kernel.org/stable/c/920f893f735e92ba3a1cd9256899a186b161928d"
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      "title": "posix-cpu-timers: Prevent UAF caused by non-leader exec() race",
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  "cveMetadata": {
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-64560",
    "datePublished": "2026-07-29T16:47:17.602Z",
    "dateReserved": "2026-07-19T15:36:31.796Z",
    "dateUpdated": "2026-09-08T08:50:45.439Z",
    "state": "PUBLISHED"
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}