Re: netfilter: Use netlink_ns_capable to verify the permisions of netlink messages

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Jul 07, 2021 at 04:05:33PM +0800, iLifetruth wrote:
> Hi, we have found that the same fix pattern of CVE-2014-0181 may not
> forward ported to some netlink-related places in the latest linux
> kernel(v5.13)
> 
> =============
> Here is the description of CVE-2014-0181:
> 
> The Netlink implementation in the Linux kernel through 3.14.1 does not
> provide a mechanism for authorizing socket operations based on the opener
> of a socket, which allows local users to bypass intended access
> restrictions and modify network configurations by using a Netlink socket
> for the (1) stdout or (2) stderr of a setuid program.
> 
> ==========
> And here is the solution to CVE-2014-0181:
> 
> To keep this from happening, replace bare capable and ns_capable calls with
> netlink_capable, netlink_net_calls and netlink_ns_capable calls. Which act
> the same as the previous calls *except they verify that the opener of the
> socket had the desired permissions as well.*
> 
> ==========
> The upstream patch commit of this vulnerability described in CVE-2014-0181
> is:
>     90f62cf30a78721641e08737bda787552428061e (committed about 7 years ago)
> 
> =========
> Capable() checks were added to these netlink-related places listed below
> in netfilter by another upstream commit:
> 4b380c42f7d00a395feede754f0bc2292eebe6e5(committed about 4 years ago)
> 
> In kernel v5.13:
>     File_1: linux/net/netfilter/nfnetlink_cthelper.c
>                        in line 424, line 623 and line 691
>     File_2: linux/net/netfilter/nfnetlink_osf.c
>                        in line 305 and line 351

These subsystems depend on nfnetlink.

nfnetlink_rcv() is called before passing the message to the
corresponding backend, e.g. nfnetlink_osf.

static void nfnetlink_rcv(struct sk_buff *skb)
{
        struct nlmsghdr *nlh = nlmsg_hdr(skb);

        if (skb->len < NLMSG_HDRLEN ||
            nlh->nlmsg_len < NLMSG_HDRLEN ||
            skb->len < nlh->nlmsg_len)
                return;

        if (!netlink_net_capable(skb, CAP_NET_ADMIN)) {
                netlink_ack(skb, nlh, -EPERM, NULL);
                return;
        }
        [...]

which is calling netlink_net_capable().

> But these checkers are still using bare capable instead of netlink_capable
> calls. So this is likely to trigger the vulnerability described in the
> CVE-2014-0181 without checking the desired permissions of the socket
> opener. Now, shall we forward port the fix pattern from the patch of
> CVE-2014-0181?
> 
> We would like to contact you to confirm this problem.

I think these capable() calls in nfnetlink_cthelper and nfnetlink_osf
are dead code that can be removed. As I explained these subsystems
stay behind nfnetlink.