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[ciqlts9_6] exec: Cleanup POSIX timers right after de_thread() - #1689
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jira VULN-219518 cve CVE-2026-98260 commit-author Hyunwoo Kim <imv4bel@gmail.com> commit acb03d3 commit-source-sha 75aa08b commit-source linux-6.18.y upstream-diff | Context due to missing unrelated change: af7bb0d ("exec: fix the racy usage of fs_struct->in_exec") commit acb03d3 upstream. A per-thread CPU timer holds a reference to the PID of the thread it is attached to and, while it is armed, its node is queued in that thread's posix_cputimers. The task is looked up by that PID. When a non-leader thread exec()s, de_thread() changes which task owns that PID. pid_task(timer->it.cpu.pid, PIDTYPE_PID) then returns NULL, but the node is still queued on tsk, which is alive. timer_lock_sighand() takes a failed lookup to mean that the node is already dequeued, so it has nothing to undo. begin_new_exec() calls posix_cpu_timers_exit(me) right after exec_task_namespaces() and that removes the leftover node, so the state normally stays invisible. But bprm->point_of_no_return is set before de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or exec_task_namespaces() fails, the task dies before it gets there. exit_itimers() then frees the k_itimer while its node is still queued, and reaping tsk later erases that freed node from the rbtree. In short: the non-leader thread B the parent timer_create(CLOCK_THREAD_CPUTIME_ID) timer_settime() arm_timer() // the node is queued on B execve() de_thread(B) exchange_tids(B, leader) // B's PID now belongs to the leader release_task(leader) __exit_signal(leader) posix_cpu_timers_exit(leader) // cleans leader's queue, not B's __unhash_process(leader) // that PID has no task anymore exec_mmap() mmap_read_lock_killable(old_mm) kill(B, SIGKILL) // -EINTR get_signal() do_exit() exit_itimers() posix_timer_delete() posix_cpu_timer_del() posix_timer_unhash_and_free() // freed while still queued wait4() release_task(B) posix_cpu_timers_exit(B) cleanup_timerqueue() timerqueue_del() // use-after-free Move the POSIX timer cleanup right after de_thread() before any of the later failure conditions brings the task into do_exit(). [ tglx: Move the cleanup right after de_thread() ] Fixes: 55e8c8e ("posix-cpu-timers: Store a reference to a pid not a task") Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com> Signed-off-by: Thomas Gleixner <tglx@kernel.org> Tested-by: Kijo Park <red993688@gmail.com> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Cc: stable@vger.kernel.org Link: https://patch.msgid.link/ao7Q8miiuLAPVnWv@v4bel Link: https://patch.msgid.link/20260911090541.627712075@kernel.org Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> (cherry picked from commit 75aa08b) Signed-off-by: Jonathan Maple <jmaple@ciq.com>
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🔍 Diffofdiffs: 1 backport differs from upstreamOpen to anyone signed in to GitHub. Report expires 2027-01-11 23:40 UTC.
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