linux-kernel - [PATCH RT] fix faulting bomb in futex_unlock

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Date:	Wed, 30 May 2007 22:52:33 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	john stultz <johnstul@...ibm.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sripathi Kodi <sripathik@...ibm.com>,
	Ulrich Drepper <drepper@...il.com>
Subject: [PATCH RT] fix faulting bomb in futex_unlock_pi64

[CC'ing Uli because John forgot to with the original patch]

As John found with futex_unlock_pi (which that patch should apply nicely
to the -rt tree), the code can get screwed up with faulting in at the
cmpxchg in futex_unlock_pi code.

>>From John's original email:

----
In looking into why the kernel was returning -EFAULT, I found the
following:

...
retry_locked:
        /*
         * To avoid races, try to do the TID -> 0 atomic transition
         * again. If it succeeds then we can return without waking
         * anyone else up:
         */
        if (!(uval & FUTEX_OWNER_DIED)) {
                pagefault_disable();
                uval = futex_atomic_cmpxchg_inatomic(uaddr,
current->pid, 0);
                pagefault_enable();
        }

        if (unlikely(uval == -EFAULT))
                goto pi_faulted;
...[snip]...
pi_faulted:
        /*
         * We have to r/w  *(int __user *)uaddr, but we can't modify it
         * non-atomically.  Therefore, if get_user below is not
         * enough, we need to handle the fault ourselves, while
         * still holding the mmap_sem.
         */
        if (attempt++) {
                ret = futex_handle_fault((unsigned long)uaddr, fshared,
                                         attempt);
                if (ret)
                        goto out_unlock;
                goto retry_locked;
        }
----


We see that uval is -EFAULT when jumping to pi_faulted. But when it goes
back to retry_locked, the uval is still -EFAULT. Which just happens to
have the FUTEX_OWNER_DIED bit set.  So we don't do the
futex_atomic_cmpxchg_inatomic call, and go to the next conditional which
just so happens to be a check of uval for -EFAULT. And guess what? it
still is!!

Way to go John in finding this!!

But, the -rt kernel has pretty much the same code for the
futex_unlock_pi64, and it has the same bug. Hence this email.

This patch is the same fix that John posted, but simply for the
futex_unlock_pi64 that exists in the RT kernel and not the mainline.

Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Index: linux-2.6-rt-work/kernel/futex.c
===================================================================
--- linux-2.6-rt-work.orig/kernel/futex.c	2007-05-30 22:29:04.000000000 -0400
+++ linux-2.6-rt-work/kernel/futex.c	2007-05-30 22:32:01.000000000 -0400
@@ -3503,6 +3503,7 @@ pi_faulted:
 			ret = -EFAULT;
 			goto out_unlock;
 		}
+		uval = 0;
 		goto retry_locked;
 	}
 


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