Re: [PATCH net] rtnetlink: Return correct error on changing device netns

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 30 Aug 2021 07:59:48 -0700 Stephen Hemminger wrote:
> On Thu, 26 Aug 2021 11:15:22 -0500
> ebiederm@xmission.com (Eric W. Biederman) wrote:
> 
> > The analysis and the fix looks good to me.
> > 
> > The code calling do_setlink is inconsistent.  One caller of do_setlink
> > passes a NULL to indicate not name has been specified.  Other callers
> > pass a string of zero bytes to indicate no name has been specified.
> > 
> > I wonder if we might want to fix the callers to uniformly pass NULL,
> > instead of a string of length zero.
> > 
> > There is a slight chance this will trigger a regression somewhere
> > because we are changing the error code but this change looks easy enough
> > to revert in the unlikely event this breaks existing userspace.
> > 
> > Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>  
> 
> This patch causes a new warning from Coverity:
> ________________________________________________________________________________________________________
> *** CID 1490867:  Null pointer dereferences  (FORWARD_NULL)
> /net/core/rtnetlink.c: 2701 in do_setlink()
> 2695     
> 2696     	/*
> 2697     	 * Interface selected by interface index but interface
> 2698     	 * name provided implies that a name change has been
> 2699     	 * requested.
> 2700     	 */
>  [...]  
> 2701     	if (ifm->ifi_index > 0 && ifname[0]) {
> 2702     		err = dev_change_name(dev, ifname);
> 2703     		if (err < 0)
> 2704     			goto errout;
> 2705     		status |= DO_SETLINK_MODIFIED;
> 2706     
> 
> Originally, the code was not accepting ifname == NULL and would
> crash. Somewhere along the way some new callers seem to have gotten
> confused.
> 
> What code is call do_setlink() with NULL as ifname, that should be fixed.

It's a false positive. There's only one caller with ifname=NULL:

static int rtnl_group_changelink(const struct sk_buff *skb,
...
			err = do_setlink(skb, dev, ifm, extack, tb, NULL, 0);
			if (err < 0)
				return err;

Which has one caller, under this condition:

		if (ifm->ifi_index == 0 && tb[IFLA_GROUP])
			return rtnl_group_changelink(skb, net, ...

condition which excludes evaluating the check in question:

	if (ifm->ifi_index > 0 && ifname[0]) {
		err = dev_change_name(dev, ifname);

Proving ifm->ifi_index has to be 0 for ifname to be NULL.