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>

3 files changed, 42 insertions, 30 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;
 }
 
 /**
diff --git a/include/linux/tty.h b/include/linux/tty.h
index e500171c745f..2a754748dd5f 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -541,8 +541,8 @@ extern void tty_audit_exit(void);
 extern void tty_audit_fork(struct signal_struct *sig);
 extern void tty_audit_tiocsti(struct tty_struct *tty, char ch);
 extern void tty_audit_push(struct tty_struct *tty);
-extern void tty_audit_push_task(struct task_struct *tsk,
-                                        uid_t loginuid, u32 sessionid);
+extern int tty_audit_push_task(struct task_struct *tsk,
+                               uid_t loginuid, u32 sessionid);
 #else
 static inline void tty_audit_add_data(struct tty_struct *tty,
                                       unsigned char *data, size_t size)
@@ -560,9 +560,10 @@ static inline void tty_audit_fork(struct signal_struct *sig)
 static inline void tty_audit_push(struct tty_struct *tty)
 {
 }
-static inline void tty_audit_push_task(struct task_struct *tsk,
-                                        uid_t loginuid, u32 sessionid)
+static inline int tty_audit_push_task(struct task_struct *tsk,
+                                      uid_t loginuid, u32 sessionid)
 {
+        return 0;
 }
 #endif
 
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;
 }