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

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Date:   Fri, 27 Apr 2018 21:07:56 -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 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.

The structure is allocated by kmalloc(), then individual fields are
initialised.  The named adding would be forced to be initialised if
it were initialised with a struct initialiser, but that's not the case.
The compiler is free to leave padding0 with whatever junk kmalloc()
left there.

Having said that, naming the padding *does* help - technically, the
compiler is allowed to put whatever it likes in the padding every time
you modify the struct.  It really needs both.

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

    - Kevin