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, 9 insertions, 16 deletions

diff --git a/kernel/audit.c b/kernel/audit.c
index a300931fc45f..8429afea37bf 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -467,23 +467,16 @@ static int audit_prepare_user_tty(pid_t pid, uid_t loginuid, u32 sessionid)
         struct task_struct *tsk;
         int err;
 
-        read_lock(&tasklist_lock);
+        rcu_read_lock();
         tsk = find_task_by_vpid(pid);
-        err = -ESRCH;
-        if (!tsk)
-                goto out;
-        err = 0;
-
-        spin_lock_irq(&tsk->sighand->siglock);
-        if (!tsk->signal->audit_tty)
-                err = -EPERM;
-        spin_unlock_irq(&tsk->sighand->siglock);
-        if (err)
-                goto out;
-
-        tty_audit_push_task(tsk, loginuid, sessionid);
-out:
-        read_unlock(&tasklist_lock);
+        if (!tsk) {
+                rcu_read_unlock();
+                return -ESRCH;
+        }
+        get_task_struct(tsk);
+        rcu_read_unlock();
+        err = tty_audit_push_task(tsk, loginuid, sessionid);
+        put_task_struct(tsk);
         return err;
 }