[PATCH] audit: Call tty_audit_push_task() outside preempt disabled region

Thomas Gleixner
2009-12-01T17:38:48Z

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>
---
drivers/char/tty_audit.c | 29 +++++++++++++++++++++--------
include/linux/tty.h | 8 ++++----
kernel/audit.c | 25 +++++++++----------------
3 files changed, 34 insertions(+), 28 deletions(-)

Index: linux-2.6-tip/drivers/char/tty_audit.c
===================================================================
--- linux-2.6-tip.orig/drivers/char/tty_audit.c
+++ linux-2.6-tip/drivers/char/tty_audit.c
@@ -188,25 +188,38 @@ void tty_audit_tiocsti(struct tty_struct
}

/**
- * 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 = NULL;
+ unsigned long flags;

- spin_lock_irq(&tsk->sighand->siglock);
- buf = tsk->signal->tty_audit_buf;
- if (buf)
+ if (!lock_task_sighand(tsk, &flags))
+ return -ESRCH;
+
+ if (tsk->signal->audit_tty && tsk->signal->tty_audit_buf) {
+ buf = tsk->signal->tty_audit_buf;
atomic_inc(&buf->count);
- spin_unlock_irq(&tsk->sighand->siglock);
+ }
+ unlock_task_sighand(tsk, &flags);
+
if (!buf)
- return;
+ return -EPERM;

mutex_lock(&buf->mutex);
tty_audit_buf_push(tsk, loginuid, sessionid, buf);
mutex_unlock(&buf->mutex);

tty_audit_buf_put(buf);
+ return 0;
}

/**
Index: linux-2.6-tip/include/linux/tty.h
===================================================================
--- linux-2.6-tip.orig/include/linux/tty.h
+++ linux-2.6-tip/include/linux/tty.h
@@ -494,8 +494,8 @@ 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)
@@ -513,8 +513,8 @@ static inline void tty_audit_fork(struct
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)
{
}
#endif
Index: linux-2.6-tip/kernel/audit.c
===================================================================
--- linux-2.6-tip.orig/kernel/audit.c
+++ linux-2.6-tip/kernel/audit.c
@@ -467,23 +467,16 @@ static int audit_prepare_user_tty(pid_t
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;
}