Subject: Re: [patch] tun: signedness bug in tun_get_user()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 15 Aug 2013 18:02:14 +0300

On Thu, Aug 15, 2013 at 05:58:40PM +0300, Michael S. Tsirkin wrote:
> On Thu, Aug 15, 2013 at 05:04:49PM +0300, Michael S. Tsirkin wrote:
> > On Thu, Aug 15, 2013 at 03:52:57PM +0300, Dan Carpenter wrote:
> > > The recent fix d9bf5f1309 "tun: compare with 0 instead of total_len" is
> > > not totally correct.  Because "len" and "sizeof()" are size_t type, that
> > > means they are never less than zero.
> > > 
> > > Signed-off-by: Dan Carpenter <[email-masked]>
> > 
> > Acked-by: Michael S. Tsirkin <[email-masked]>
> 
> Alternatively how about we revert the original patch?
> This is not the only issue it introduced and it doesn't
> actually fix any bugs.
> 
> > > 
> > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > > index af987f0..7ed13cc 100644
> > > --- a/drivers/net/tun.c
> > > +++ b/drivers/net/tun.c
> > > @@ -977,8 +977,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > >  	u32 rxhash;
> > >  
> > >  	if (!(tun->flags & TUN_NO_PI)) {
> > > -		if ((len -= sizeof(pi)) < 0)
> > > +		if (len < sizeof(pi))
> > >  			return -EINVAL;
> > > +		len -= sizeof(pi);
> > >  
> > >  		if (memcpy_fromiovecend((void *)&pi, iv, 0, sizeof(pi)))
> > >  			return -EFAULT;
> > > @@ -986,8 +987,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> > >  	}
> > >  
> > >  	if (tun->flags & TUN_VNET_HDR) {
> > > -		if ((len -= tun->vnet_hdr_sz) < 0)
> > > +		if (len < tun->vnet_hdr_sz)
> > >  			return -EINVAL;

And to be even more explicit, this still doesn't handle the
case vnet_hdr_sz < 0 properly.

> > > +		len -= tun->vnet_hdr_sz;
> > >  
> > >  		if (memcpy_fromiovecend((void *)&gso, iv, offset, sizeof(gso)))
> > >  			return -EFAULT;