[PATCH v4] tty: add missing rcu_read_lock for task_pgrp

[Patrick Donnelly <batrick@batbytes.com>]

task_pgrp requires an rcu or tasklist lock to be obtained if the returned pid
is to be dereferenced, which kill_pgrp does. Obtain an RCU lock for the
duration of use.

Signed-off-by: Patrick Donnelly <batrick@batbytes.com>
---
 drivers/tty/n_tty.c  | 15 ++++++++++++---
 drivers/tty/tty_io.c | 17 ++++++++++++-----
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index c9c27f6..de67b2c 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2137,6 +2137,8 @@ extern ssize_t redirected_tty_write(struct file *, const char __user *,
 
 static int job_control(struct tty_struct *tty, struct file *file)
 {
+	struct pid *pgrp;
+
 	/* Job control check -- must be done at start and after
 	   every sleep (POSIX.1 7.1.1.4). */
 	/* NOTE: not yet done after every sleep pending a thorough
@@ -2146,18 +2148,25 @@ static int job_control(struct tty_struct *tty, struct file *file)
 	    current->signal->tty != tty)
 		return 0;
 
+	rcu_read_lock();
+	pgrp = task_pgrp(current);
+
 	spin_lock_irq(&tty->ctrl_lock);
 	if (!tty->pgrp)
 		printk(KERN_ERR "n_tty_read: no tty->pgrp!\n");
-	else if (task_pgrp(current) != tty->pgrp) {
+	else if (pgrp != tty->pgrp) {
 		spin_unlock_irq(&tty->ctrl_lock);
-		if (is_ignored(SIGTTIN) || is_current_pgrp_orphaned())
+		if (is_ignored(SIGTTIN) || is_current_pgrp_orphaned()) {
+			rcu_read_unlock();
 			return -EIO;
-		kill_pgrp(task_pgrp(current), SIGTTIN, 1);
+		}
+		kill_pgrp(pgrp, SIGTTIN, 1);
+		rcu_read_unlock();
 		set_thread_flag(TIF_SIGPENDING);
 		return -ERESTARTSYS;
 	}
 	spin_unlock_irq(&tty->ctrl_lock);
+	rcu_read_unlock();
 	return 0;
 }
 
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 57fc6ee..6bdfb98 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -388,33 +388,40 @@ EXPORT_SYMBOL_GPL(tty_find_polling_driver);
 int tty_check_change(struct tty_struct *tty)
 {
 	unsigned long flags;
+	struct pid *pgrp;
 	int ret = 0;
 
 	if (current->signal->tty != tty)
 		return 0;
 
+	rcu_read_lock();
+	pgrp = task_pgrp(current);
+
 	spin_lock_irqsave(&tty->ctrl_lock, flags);
 
 	if (!tty->pgrp) {
 		printk(KERN_WARNING "tty_check_change: tty->pgrp == NULL!\n");
 		goto out_unlock;
 	}
-	if (task_pgrp(current) == tty->pgrp)
+	if (pgrp == tty->pgrp)
 		goto out_unlock;
 	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+
 	if (is_ignored(SIGTTOU))
-		goto out;
+		goto out_rcuunlock;
 	if (is_current_pgrp_orphaned()) {
 		ret = -EIO;
-		goto out;
+		goto out_rcuunlock;
 	}
-	kill_pgrp(task_pgrp(current), SIGTTOU, 1);
+	kill_pgrp(pgrp, SIGTTOU, 1);
+	rcu_read_unlock();
 	set_thread_flag(TIF_SIGPENDING);
 	ret = -ERESTARTSYS;
-out:
 	return ret;
 out_unlock:
 	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+out_rcuunlock:
+	rcu_read_unlock();
 	return ret;
 }
 
-- 
Patrick Donnelly