lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Tue, 27 Oct 2009 09:39:03 -0400
From:	Gregory Haskins <>
To:	Gleb Natapov <>
CC:	Gregory Haskins <>,,
Subject: Re: [KVM PATCH v3 1/3] KVM: fix race in irq_routing logic

Gleb Natapov wrote:
> On Mon, Oct 26, 2009 at 12:21:57PM -0400, Gregory Haskins wrote:
>> The current code suffers from the following race condition:
>> thread-1                                    thread-2
>> -----------------------------------------------------------
>> kvm_set_irq() {
>>    rcu_read_lock()
>>    irq_rt = rcu_dereference(table);
>>    rcu_read_unlock();
>>                                        kvm_set_irq_routing() {
>>                                           mutex_lock();
>>                                           irq_rt = table;
>>                                           rcu_assign_pointer();
>>                                           mutex_unlock();
>>                                           synchronize_rcu();
>>                                           kfree(irq_rt);
>>    irq_rt->entry->set(); /* bad */
> This is not what happens. irq_rt is never accessed outside read-side
> critical section.

Sorry, I was generalizing to keep the comments short.  I figured it
would be clear what I was actually saying, but realize in retrospect
that I was a little ambiguous.

Yes, irq_rt is not accessed outside the RSCS.  However, the entry
pointers stored in the irq_rt->map are, and this is equally problematic

In this particular case we seem to never delete entries at run-time once
they are established.  Therefore, while perhaps sloppy, its technically
safe to leave them unprotected from this perspective.  The issue is more
related to shutdown since a kvm_set_irq() caller could be within the
aforementioned race-region and call entry->set() after the guest is
gone.  Or did I miss something?

> Data is copied from irq_rt onto the stack and this copy is accessed
> outside critical section.

As mentioned above, I do not believe this really protect us.  And even
if it did, the copy is just a work-around to avoid sleeping within the
standard RCU RSCS, which is what SRCU is designed for.  So rather than
inventing an awkward two-phased stack based solution, it's better to
reuse the provided tools, IMO.

To flip it around:  Is there any reason why an SRCU would not work here,
and thus we were forced to use something like the stack-copy approach?

Kind Regards,

Download attachment "signature.asc" of type "application/pgp-signature" (268 bytes)

Powered by blists - more mailing lists