Re: selinux_netlink_send changes program behavior

[Paul Moore]

On Sat, Apr 25, 2020 at 8:01 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Sat, Apr 25, 2020 at 1:42 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.
> >
> > Your original email gave the impression that there was a big in the non-SELinux case; if that is not the case my response changes.
>
>
> There is no bug... Well, there is a crash, but it is somewhere in the
> routing subsystem and is caused by the contents of the second netlink
> message. This is totally unrelated to this SELinux check and that
> crash is totally reproducible with SELinux as well if we just don't
> send the first empty message.
> The crux is really a difference in behavior in SELinux and non-SELinux cases.

We're taking a closer look from the SELinux side of things.

-- 
paul moore
www.paul-moore.com