ptrace: fix possible zombie leak on PTRACE_DETACH

When ptrace_detach() takes tasklist, the tracee can be SIGKILL'ed.  If it
has already passed exit_notify() we can leak a zombie, because a) ptracing
disables the auto-reaping logic, and b) ->real_parent was not notified
about the child's death.

ptrace_detach() should follow the ptrace_exit's logic, change the code
accordingly.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Tested-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/ptrace.h

@@ -94,6 +94,7 @@ extern void ptrace_notify(int exit_code);
 extern void __ptrace_link(struct task_struct *child,
 			  struct task_struct *new_parent);
 extern void __ptrace_unlink(struct task_struct *child);
+extern int __ptrace_detach(struct task_struct *tracer, struct task_struct *p);
 extern void ptrace_fork(struct task_struct *task, unsigned long clone_flags);
 #define PTRACE_MODE_READ   1
 #define PTRACE_MODE_ATTACH 2

kernel/ptrace.c

@@ -237,6 +237,8 @@ int ptrace_attach(struct task_struct *task)
 
 int ptrace_detach(struct task_struct *child, unsigned int data)
 {
+	int dead = 0;
+
 	if (!valid_signal(data))
 		return -EIO;
 
@@ -244,18 +246,21 @@ int ptrace_detach(struct task_struct *child, unsigned int data)
 	ptrace_disable(child);
 	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
 
-	/* protect against de_thread()->release_task() */
 	write_lock_irq(&tasklist_lock);
+	/* protect against de_thread()->release_task() */
 	if (child->ptrace) {
 		child->exit_code = data;
 
-		__ptrace_unlink(child);
+		dead = __ptrace_detach(current, child);
 
 		if (!child->exit_state)
 			wake_up_process(child);
 	}
 	write_unlock_irq(&tasklist_lock);
 
+	if (unlikely(dead))
+		release_task(child);
+
 	return 0;
 }