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

[Julian Anastasov]

	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 <yepeilin.cs@xxxxxxxxx> 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.

	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 :)

> @@ -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 <ja@xxxxxx>