From cdd812d0683dee14ead02c9eded568685e61b23f Mon Sep 17 00:00:00 2001 From: Daehyeon Ko <4ncienth@gmail.com> Date: Mon, 31 Aug 2026 09:12:21 +0900 Subject: exit: hold a reference to thread_pid across proc_flush_pid Commit 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") removed the reference around proc_flush_pid(). It assumed that free_pids(post.pids) at the end of release_task() would keep thread_pid alive until then. That assumption is wrong. __change_pid() only records a detached PID in post.pids when pid_has_task() is false for every PIDTYPE. If another task still uses the exiting task's PID as its process group or session ID, __unhash_process() removes the exiting task's PIDTYPE_PID link but leaves the PID out of post.pids. release_task() therefore holds no reference to it after dropping tasklist_lock. The other task can then remove the remaining PIDTYPE links. Its free_pids() call schedules delayed_put_pid(), and the RCU callback can free the PID before the first release_task() reaches proc_flush_pid(). An unprivileged reproducer races wait4(-1) against setsid() to trigger this ordering. Three of three fresh v7.2 KASAN boots reported: BUG: KASAN: slab-use-after-free in proc_invalidate_siblings_dcache+0x3e2/0x3f0 Read of size 8 by task h7_pid_reaper/1921 Call Trace: proc_invalidate_siblings_dcache release_task wait_consider_task __do_wait do_wait kernel_wait4 Freed by task 0: kmem_cache_free put_pid delayed_put_pid rcu_core Last potentially related work creation: __call_rcu_common free_pids ksys_setsid KASAN identified a 144-byte object from the pid cache and located the bad read 80 bytes into the freed object, matching pid->inodes. With an explicit reference, three of three fresh boots completed without a KASAN report. The concurrent RCU callback dropped its reference while proc_flush_pid() was protected, and the balancing put_pid() performed the final free afterward. Take a reference before __unhash_process() clears p->thread_pid and release it after proc_flush_pid() completes. A tested source reproducer is available privately on request. No controlled read or write, information leak, or privilege escalation is claimed. The mainline patch applies directly to v6.19.y and newer; v6.16.y through v6.18.y need a context-adjusted backport. Fixes: 0a36bad01731 ("release_task: kill the no longer needed get/put_pid(thread_pid)") Reported-by: syzbot+0aee5e8066eddbbe7397@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=0aee5e8066eddbbe7397 Reported-by: syzbot+e8b3520b53e78e90034e@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?extid=e8b3520b53e78e90034e Cc: stable@vger.kernel.org # see patch description, needs adjustments for 6.16.y-6.18.y Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Link: https://patch.msgid.link/20260831001221.3755948-1-4ncienth@gmail.com Acked-by: Oleg Nesterov Reviewed-by: Bradley Morgan Signed-off-by: Christian Brauner (Amutable) --- kernel/exit.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 182c06671c78..e12072f1507c 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -261,8 +261,11 @@ repeat: pidfs_exit(p); cgroup_task_release(p); - /* Retrieve @thread_pid before __unhash_process() may set it to NULL. */ - thread_pid = task_pid(p); + /* + * Pin @thread_pid before __unhash_process() clears it. The last + * PIDTYPE detach can otherwise free it before proc_flush_pid(). + */ + thread_pid = get_pid(task_pid(p)); write_lock_irq(&tasklist_lock); ptrace_release_task(p); @@ -291,8 +294,8 @@ repeat: } write_unlock_irq(&tasklist_lock); - /* @thread_pid can't go away until free_pids() below */ proc_flush_pid(thread_pid); + put_pid(thread_pid); exit_cred_namespaces(p); add_device_randomness(&p->se.sum_exec_runtime, sizeof(p->se.sum_exec_runtime)); -- cgit