GIT: unionfs2-2.6.27.y: futex: Handle futex value corruption gracefully

[Erez Zadok]

commit 268ec2564310a552681bcbfb412c35804c06efad
Author: Thomas Gleixner <tglx at linutronix.de>
Date:   Wed Feb 3 09:33:05 2010 +0100

    futex: Handle futex value corruption gracefully
    
    commit 59647b6ac3050dd964bc556fe6ef22f4db5b935c upstream.
    
    The WARN_ON in lookup_pi_state which complains about a mismatch
    between pi_state->owner->pid and the pid which we retrieved from the
    user space futex is completely bogus.
    
    The code just emits the warning and then continues despite the fact
    that it detected an inconsistent state of the futex. A conveniant way
    for user space to spam the syslog.
    
    Replace the WARN_ON by a consistency check. If the values do not match
    return -EINVAL and let user space deal with the mess it created.
    
    This also fixes the missing task_pid_vnr() when we compare the
    pi_state->owner pid with the futex value.
    
    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 ec84da5..57335df 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -533,8 +533,25 @@ lookup_pi_state(u32 uval, struct futex_hash_bucket *hb,
 				return -EINVAL;
 
 			WARN_ON(!atomic_read(&pi_state->refcount));
-			WARN_ON(pid && pi_state->owner &&
-				pi_state->owner->pid != pid);
+
+			/*
+			 * When pi_state->owner is NULL then the owner died
+			 * and another waiter is on the fly. pi_state->owner
+			 * is fixed up by the task which acquires
+			 * pi_state->rt_mutex.
+			 *
+			 * We do not check for pid == 0 which can happen when
+			 * the owner died and robust_list_exit() cleared the
+			 * TID.
+			 */
+			if (pid && pi_state->owner) {
+				/*
+				 * Bail out if user space manipulated the
+				 * futex value.
+				 */
+				if (pid != task_pid_vnr(pi_state->owner))
+					return -EINVAL;
+			}
 
 			atomic_inc(&pi_state->refcount);
 			*ps = pi_state;