[PATCH] fork: fix race in setting child's pgrp and tty

In fork, child should recopy parent's pgrp/tty after it has tasklist_lock.
Otherwise following a setpgid() on the parent, *after* copy_signal(), the
child will own a stale pgrp (which may be reused); (eg.  if copy_mm()
sleeps a long while due to memory pressure).  Similar issue for the tty.
Signed-off-by: Oren Laadan <orenl@cs.columbia.edu>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/fork.c

@@ -811,9 +811,6 @@ static inline int copy_signal(unsigned long clone_flags, struct task_struct * ts
 	sig->it_prof_expires = cputime_zero;
 	sig->it_prof_incr = cputime_zero;
 
-	sig->tty = current->signal->tty;
-	sig->pgrp = process_group(current);
-	sig->session = current->signal->session;
 	sig->leader = 0;	/* session leadership doesn't inherit */
 	sig->tty_old_pgrp = 0;
 
@@ -1136,15 +1133,15 @@ static task_t *copy_process(unsigned long clone_flags,
 	attach_pid(p, PIDTYPE_PID, p->pid);
 	attach_pid(p, PIDTYPE_TGID, p->tgid);
 	if (thread_group_leader(p)) {
+		p->signal->tty = current->signal->tty;
+		p->signal->pgrp = process_group(current);
+		p->signal->session = current->signal->session;
 		attach_pid(p, PIDTYPE_PGID, process_group(p));
 		attach_pid(p, PIDTYPE_SID, p->signal->session);
 		if (p->pid)
 			__get_cpu_var(process_counts)++;
 	}
 
-	if (!current->signal->tty && p->signal->tty)
-		p->signal->tty = NULL;
-
 	nr_threads++;
 	total_forks++;
 	write_unlock_irq(&tasklist_lock);