linux-kernel - Re: KMSAN: uninit-value in _copy_to

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Date:   Thu, 7 Jun 2018 19:04:49 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     syzbot <syzbot+87cfa083e727a224754b@...kaller.appspotmail.com>,
        avagin@...nvz.org, davem@...emloft.net, dingtianhong@...wei.com,
        edumazet@...gle.com, elena.reshetova@...el.com,
        jasowang@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, matthew@...systems.ca,
        mingo@...nel.org, netdev@...r.kernel.org, pabeni@...hat.com,
        syzkaller-bugs@...glegroups.com,
        virtualization@...ts.linux-foundation.org, willemb@...gle.com
Subject: Re: KMSAN: uninit-value in _copy_to_iter (2)

On Thu, Jun 07, 2018 at 08:59:06PM +0300, Michael S. Tsirkin wrote:
> On Thu, Jun 07, 2018 at 06:43:55PM +0100, Al Viro wrote:
> > On Thu, Jun 07, 2018 at 06:38:48PM +0300, Michael S. Tsirkin wrote:
> > > #syz test: https://github.com/google/kmsan.git/master d2d741e5d1898dfde1a75ea3d29a9a3e2edf0617
> > > 
> > > Subject: vhost: fix info leak
> > > 
> > > Fixes: CVE-2018-1118
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > ---
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index f0be5f35ab28..9beefa6ed1ce 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -2345,6 +2345,9 @@ struct vhost_msg_node *vhost_new_msg(struct vhost_virtqueue *vq, int type)
> > >  	struct vhost_msg_node *node = kmalloc(sizeof *node, GFP_KERNEL);
> > >  	if (!node)
> > >  		return NULL;
> > > +
> > > +	/* Make sure all padding within the structure is initialized. */
> > > +	memset(&node->msg, 0, sizeof node->msg);
> > 
> > Umm...  Maybe kzalloc(), then?  You have
> > 
> > struct vhost_msg_node {
> >   struct vhost_msg msg;
> >   struct vhost_virtqueue *vq;
> >   struct list_head node;
> > };
> > 
> > and that's what, 68 bytes in msg, then either 4 bytes pointer or
> > 4 bytes padding + 8 bytes pointer, then two pointers?  How much
> > does explicit partial memset() save you here?
> 
> Yes but 0 isn't a nop here so if this struct is used without
> a sensible initialization, it will crash elsewhere.
> I prefer KASAN to catch such uses.
> 
> 
> > >  	node->vq = vq;
> > >  	node->msg.type = type;

IDGI - what would your variant catch that kzalloc + 2 assignments won't?
Accesses to uninitialized ->node?  Because that's the only difference in
what is and is not initialized between those variants...