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Date:   Thu, 27 Sep 2018 19:50:24 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     stefanha@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH net V2] vhost-vsock: fix use after free

On Fri, Sep 28, 2018 at 07:37:37AM +0800, Jason Wang wrote:
> 
> 
> On 2018年09月28日 01:04, Michael S. Tsirkin wrote:
> > On Thu, Sep 27, 2018 at 08:22:04PM +0800, Jason Wang wrote:
> > > The access of vsock is not protected by vhost_vsock_lock. This may
> > > lead to use after free since vhost_vsock_dev_release() may free the
> > > pointer at the same time.
> > > 
> > > Fix this by holding the lock during the access.
> > > 
> > > Reported-by:syzbot+e3e074963495f92a89ed@...kaller.appspotmail.com
> > > Fixes: 16320f363ae1 ("vhost-vsock: add pkt cancel capability")
> > > Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
> > > Cc: Stefan Hajnoczi<stefanha@...hat.com>
> > > Signed-off-by: Jason Wang<jasowang@...hat.com>
> > Wow is that really the best we can do?
> 
> For net/stable, probably yes.
> 
> >   A global lock on a data path
> > operation?
> 
> It's already there,

&vhost_vsock_lock? were is it takes on data path?

> and the patch only increase the critical section.
> 
> >   Granted use after free is nasty but Stefan said he sees
> > a way to fix it using a per socket refcount. He's on vacation
> > until Oct 4 though ...
> > 
> 
> Stefan has acked the pacth, so I think it's ok? We can do optimization for
> -next on top.
> 
> Thanks


Well on high SMP serializing can drop performance as much as x100 so I'm
not sure it's appropriate - seems to fix a bug but can introduce a
regression. Let's see how does a proper fix look first?

-- 
MST

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