netdev - Re: [PATCH net] vhost: Use kzalloc() to allocate vhost_msg

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Date:   Fri, 27 Apr 2018 21:51:06 -0400
From:   Kevin Easton <kevin@...rana.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH net] vhost: Use kzalloc() to allocate vhost_msg_node

On Fri, Apr 27, 2018 at 09:07:56PM -0400, Kevin Easton wrote:
> On Fri, Apr 27, 2018 at 07:05:45PM +0300, Michael S. Tsirkin wrote:
> > On Fri, Apr 27, 2018 at 11:45:02AM -0400, Kevin Easton wrote:
> > > The struct vhost_msg within struct vhost_msg_node is copied to userspace,
> > > so it should be allocated with kzalloc() to ensure all structure padding
> > > is zeroed.
> > > 
> > > Signed-off-by: Kevin Easton <kevin@...rana.org>
> > > Reported-by: syzbot+87cfa083e727a224754b@...kaller.appspotmail.com
> > 
> > Does it help if a patch naming the padding is applied,
> > and then we init just the relevant field?
> > Just curious.
> 
> No, I don't believe that is sufficient to fix the problem.

Scratch that, somehow I missed the "..and then we init just the
relevant field" part, sorry.

There's still the padding after the vhost_iotlb_msg to consider.  It's
named in the union but I don't think that's guaranteed to be initialised
when the iotlb member of the union is used to initialise things.

> I didn't name the padding in my original patch because I wasn't sure
> if the padding actually exists on 32 bit architectures?

This might still be a concern, too?

At the end of the day, zeroing 96 bytes (the full size of vhost_msg_node)
is pretty quick.

    - Kevin