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

[Oren Laadan <orenl@cs.columbia.edu>]

[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>
---

diff --git a/kernel/fork.c b/kernel/fork.c
index fb8572a..059e71f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1055,6 +1055,15 @@ static task_t *copy_process(unsigned lon
 			!cpu_online(task_cpu(p))))
 		set_task_cpu(p, smp_processor_id());

+	/* 
+	 * signal->{prgp,tty} may have changed since we had copied them;
+	 * pgrp may have been freed -- and reused -- since then  [orenl]
+	 */
+	if (p->signal != current->signal) {
+		p->signal->tty = current->signal->tty;
+		p->signal->pgrp = process_group(current);
+	}
+
 	/*
 	 * Check for pending SIGKILL! The new thread should not be allowed
 	 * to slip out of an OOM kill. (or normal SIGKILL.)
-
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