Subject: Re: [Linux-kernel-mentees] [PATCH net] ipvs: Fix uninit-value in do_ip_vs_set_ctl()

From: Peilin Ye <[email-masked]>
Date: Tue, 11 Aug 2020 03:19:14 -0400

On Tue, Aug 11, 2020 at 09:58:46AM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Tue, 11 Aug 2020, Peilin Ye wrote:
> 
> > On Mon, Aug 10, 2020 at 08:57:19PM -0700, Cong Wang wrote:
> > > On Mon, Aug 10, 2020 at 3:10 PM Peilin Ye <[email-masked]> wrote:
> > > >
> > > > do_ip_vs_set_ctl() is referencing uninitialized stack value when `len` is
> > > > zero. Fix it.
> > > 
> > > Which exact 'cmd' is it here?
> > > 
> > > I _guess_ it is one of those uninitialized in set_arglen[], which is 0.
> > 
> > Yes, it was `IP_VS_SO_SET_NONE`, implicitly initialized to zero.
> > 
> > > But if that is the case, should it be initialized to
> > > sizeof(struct ip_vs_service_user) instead because ip_vs_copy_usvc_compat()
> > > is called anyway. Or, maybe we should just ban len==0 case.
> > 
> > I see. I think the latter would be easier, but we cannot ban all of
> > them, since the function does something with `IP_VS_SO_SET_FLUSH`, which
> > is a `len == 0` case.
> > 
> > Maybe we do something like this?
> 
> 	Yes, only IP_VS_SO_SET_FLUSH uses len 0. We can go with
> this change but you do not need to target net tree, as the
> problem is not fatal net-next works too. What happens is
> that we may lookup services with random search keys which
> is harmless.

I see, I'll target net-next instead.

> 	Another option is to add new block after this one:
> 
>         } else if (cmd == IP_VS_SO_SET_TIMEOUT) {
>                 /* Set timeout values for (tcp tcpfin udp) */
>                 ret = ip_vs_set_timeout(ipvs, (struct ip_vs_timeout_user *)arg);
>                 goto out_unlock;
>         }
> 
> 	such as:
> 
> 	} else if (!len) {
> 		/* No more commands with len=0 below */
> 		ret = -EINVAL;
> 		goto out_unlock;
> 	}
> 
> 	It give more chance for future commands to use len=0
> but the drawback is that the check happens under mutex. So, I'm
> fine with both versions, it is up to you to decide :)

Ah, this seems much cleaner. I'll send v2 soon, thank you!

Peilin Ye

> > @@ -2432,6 +2432,8 @@ do_ip_vs_set_ctl(struct sock *sk, int cmd, void __user *user, unsigned int len)
> > 
> >  	if (cmd < IP_VS_BASE_CTL || cmd > IP_VS_SO_SET_MAX)
> >  		return -EINVAL;
> > +	if (len == 0 && cmd != IP_VS_SO_SET_FLUSH)
> > +		return -EINVAL;
> >  	if (len != set_arglen[CMDID(cmd)]) {
> >  		IP_VS_DBG(1, "set_ctl: len %u != %u\n",
> >  			  len, set_arglen[CMDID(cmd)]);
> > @@ -2547,9 +2549,6 @@ do_ip_vs_set_ctl(struct sock *sk, int cmd, void __user *user, unsigned int len)
> >  		break;
> >  	case IP_VS_SO_SET_DELDEST:
> >  		ret = ip_vs_del_dest(svc, &udest);
> > -		break;
> > -	default:
> > -		ret = -EINVAL;
> >  	}
> > 
> >    out_unlock:
> 
> Regards
> 
> --
> Julian Anastasov <[email-masked]>