audit: Call tty_audit_push_task() outside preempt disabled

While auditing all tasklist_lock read_lock sites I stumbled over the
following call chain:

audit_prepare_user_tty()
  read_lock(&tasklist_lock);
  tty_audit_push_task();
     mutex_lock(&buf->mutex);

     --> buf->mutex is locked with preemption disabled.

Solve this by acquiring a reference to the task struct under
rcu_read_lock and call tty_audit_push_task outside of the preempt
disabled region.

Move all code which needs to be protected by sighand lock into
tty_audit_push_task() and use lock/unlock_sighand as we do not hold
tasklist_lock.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 28 insertions, 10 deletions

diff --git a/drivers/char/tty_audit.c b/drivers/char/tty_audit.c
index 1b8ee590b4ca..f64582b0f623 100644
--- a/drivers/char/tty_audit.c
+++ b/drivers/char/tty_audit.c
@@ -188,25 +188,43 @@ void tty_audit_tiocsti(struct tty_struct *tty, char ch)
 }
 
 /**
- *      tty_audit_push_task     -       Flush task's pending audit data
+ * tty_audit_push_task  -       Flush task's pending audit data
+ * @tsk:                task pointer
+ * @loginuid:           sender login uid
+ * @sessionid:          sender session id
+ *
+ * Called with a ref on @tsk held. Try to lock sighand and get a
+ * reference to the tty audit buffer if available.
+ * Flush the buffer or return an appropriate error code.
  */
-void tty_audit_push_task(struct task_struct *tsk, uid_t loginuid, u32 sessionid)
+int tty_audit_push_task(struct task_struct *tsk, uid_t loginuid, u32 sessionid)
 {
-        struct tty_audit_buf *buf;
+        struct tty_audit_buf *buf = ERR_PTR(-EPERM);
+        unsigned long flags;
 
-        spin_lock_irq(&tsk->sighand->siglock);
-        buf = tsk->signal->tty_audit_buf;
-        if (buf)
-                atomic_inc(&buf->count);
-        spin_unlock_irq(&tsk->sighand->siglock);
-        if (!buf)
-                return;
+        if (!lock_task_sighand(tsk, &flags))
+                return -ESRCH;
+
+        if (tsk->signal->audit_tty) {
+                buf = tsk->signal->tty_audit_buf;
+                if (buf)
+                        atomic_inc(&buf->count);
+        }
+        unlock_task_sighand(tsk, &flags);
+
+        /*
+         * Return 0 when signal->audit_tty set
+         * but tsk->signal->tty_audit_buf == NULL.
+         */
+        if (!buf || IS_ERR(buf))
+                return PTR_ERR(buf);
 
         mutex_lock(&buf->mutex);
         tty_audit_buf_push(tsk, loginuid, sessionid, buf);
         mutex_unlock(&buf->mutex);
 
         tty_audit_buf_put(buf);
+        return 0;
 }
 
 /**