GIT: unionfs2-2.6.27.y: futex: Handle user space corruption gracefully

[Erez Zadok]

commit 873b48a4cf4a64dd77faef25ddcf1651cef49a32
Author: Thomas Gleixner <tglx at linutronix.de>
Date:   Tue Feb 2 11:40:27 2010 +0100

    futex: Handle user space corruption gracefully
    
    commit 51246bfd189064079c54421507236fd2723b18f3 upstream.
    
    If the owner of a PI futex dies we fix up the pi_state and set
    pi_state->owner to NULL. When a malicious or just sloppy programmed
    user space application sets the futex value to 0 e.g. by calling
    pthread_mutex_init(), then the futex can be acquired again. A new
    waiter manages to enqueue itself on the pi_state w/o damage, but on
    unlock the kernel dereferences pi_state->owner and oopses.
    
    Prevent this by checking pi_state->owner in the unlock path. If
    pi_state->owner is not current we know that user space manipulated the
    futex value. Ignore the mess and return -EINVAL.
    
    This catches the above case and also the case where a task hijacks the
    futex by setting the tid value and then tries to unlock it.
    
    Reported-by: Jermome Marchand <jmarchan at redhat.com>
    Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
    Acked-by: Darren Hart <dvhltc at us.ibm.com>
    Acked-by: Peter Zijlstra <a.p.zijlstra at chello.nl>
    Signed-off-by: Greg Kroah-Hartman <gregkh at suse.de>

diff --git a/kernel/futex.c b/kernel/futex.c
index 57335df..02d07e4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -647,6 +647,13 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
 	if (!pi_state)
 		return -EINVAL;
 
+	/*
+	 * If current does not own the pi_state then the futex is
+	 * inconsistent and user space fiddled with the futex value.
+	 */
+	if (pi_state->owner != current)
+		return -EINVAL;
+
 	spin_lock(&pi_state->pi_mutex.wait_lock);
 	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);