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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 15, 2013 at 06:02:14PM +0300, Michael S. Tsirkin wrote:
> 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 <dan.carpenter@oracle.com>
> > > 
> > > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> > 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.

Hmm ENOCOFFEE.
User can't make vnet_hdr_sz < 0 - we already catch that.
So let's apply Dan's patch, it does fix all issues after all.
Sorry about the noise.

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