Re: selinux_netlink_send changes program behavior

[Dmitry Vyukov]

On Fri, Apr 24, 2020 at 11:51 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 24, 2020 at 4:27 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > Hi SELinux maintainers,
> >
> > We've hit a case where a developer wasn't able to reproduce a kernel
> > bug, it turned out to be a difference in behavior between SELinux and
> > non-SELinux kernels.
> > Condensed version: a program does sendmmsg on netlink socket with 2
> > mmsghdr's, first is completely empty/zeros, second contains some
> > actual payload. Without SELinux the first mmsghdr is treated as no-op
> > and the kernel processes the second one (triggers bug). However the
> > SELinux hook does:
> >
> > static int selinux_netlink_send(struct sock *sk, struct sk_buff *skb)
> > {
> >     if (skb->len < NLMSG_HDRLEN) {
> >         err = -EINVAL;
> >         goto out;
> >     }
> >
> > and fails processing on the first empty mmsghdr (does not happen
> > without SELinux).
> >
> > Is this difference in behavior intentional/acceptable/should be fixed?
>
> From a practical perspective, SELinux is always going to need to do a
> length check as it needs to peek into the netlink message header for
> the message type so it can map that to the associated SELinux
> permissions.  So in that sense, the behavior is intentional and
> desired; however from a bug-for-bug compatibility perspective ... not
> so much.
>
> Ultimately, my it's-Friday-and-it's-been-a-long-week-ending-in-a-long-day
> thought is that this was a buggy operation to begin with and the bug
> was just caught in different parts of the kernel, depending on how it
> was configured.  It may not be ideal, but I can think of worse things
> (and arguably SELinux is doing the Right Thing).

+netlink maintainers for intended semantics of empty netlink messages

If it's a bug, or intended behavior depends on the intended
behavior... which I assume is not documented anywhere officially.
However, most of the netlink families use netlink_rcv_skb, which does:

int netlink_rcv_skb(struct sk_buff *skb, int (*cb)(struct sk_buff *,
                           struct nlmsghdr *,
                           struct netlink_ext_ack *))
{
    ...
    while (skb->len >= nlmsg_total_size(0)) {
    ...
       skb_pull(skb, msglen);
    }
    return 0;
}

1. How intentional is this while loop logic vs sloppy error checking?
2. netlink_rcv_skb seems to be able to handle 2+ messages in the same
skb, while selinux_netlink_send only checks the first one... so can I
skip SELinux checks by putting a malicious message after a permitted
one?..