n_tty: Lock access to tty->pgrp for POSIX job control

Concurrent access to tty->pgrp must be protected with tty->ctrl_lock.
Also, as noted in the comments, reading current->signal->tty is
safe because either,
  1) current->signal->tty is assigned by current, or
  2) current->signal->tty is set to NULL.

NB: for reference, tty_check_change() implements a similar POSIX
check for the ioctls corresponding to tcflush(), tcdrain(),
tcsetattr(), tcsetpgrp(), tcflow() and tcsendbreak().

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 18 insertions, 16 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 61f1bc97ccd9..68865d9af8a0 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1719,10 +1719,9 @@ extern ssize_t redirected_tty_write(struct file *, const char __user *,
  *      and if appropriate send any needed signals and return a negative
  *      error code if action should be taken.
  *
- *      FIXME:
- *      Locking: None - redirected write test is safe, testing
- *      current->signal should possibly lock current->sighand
- *      pgrp locking ?
+ *      Locking: redirected write test is safe
+ *               current->signal->tty check is safe
+ *               ctrl_lock to safely reference tty->pgrp
  */
 
 static int job_control(struct tty_struct *tty, struct file *file)
@@ -1732,19 +1731,22 @@ static int job_control(struct tty_struct *tty, struct file *file)
         /* NOTE: not yet done after every sleep pending a thorough
            check of the logic of this change. -- jlc */
         /* don't stop on /dev/console */
-        if (file->f_op->write != redirected_tty_write &&
-            current->signal->tty == tty) {
-                if (!tty->pgrp)
-                        printk(KERN_ERR "n_tty_read: no tty->pgrp!\n");
-                else if (task_pgrp(current) != tty->pgrp) {
-                        if (is_ignored(SIGTTIN) ||
-                            is_current_pgrp_orphaned())
-                                return -EIO;
-                        kill_pgrp(task_pgrp(current), SIGTTIN, 1);
-                        set_thread_flag(TIF_SIGPENDING);
-                        return -ERESTARTSYS;
-                }
+        if (file->f_op->write == redirected_tty_write ||
+            current->signal->tty != tty)
+                return 0;
+
+        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) {
+                spin_unlock_irq(&tty->ctrl_lock);
+                if (is_ignored(SIGTTIN) || is_current_pgrp_orphaned())
+                        return -EIO;
+                kill_pgrp(task_pgrp(current), SIGTTIN, 1);
+                set_thread_flag(TIF_SIGPENDING);
+                return -ERESTARTSYS;
         }
+        spin_unlock_irq(&tty->ctrl_lock);
         return 0;
 }