[urgent] netlink: Only check file credentials for implicit destinations

From: "Eric W. Biederman" <ebiederm@xmission.com>

It was possible to get a setuid root or setcap executable to write to
it's stdout or stderr (which has been set made a netlink socket) and
inadvertently reconfigure the networking stack.

To prevent this we check that both the creator of the socket and
the currentl applications has permission to reconfigure the network
stack.

Unfortunately this breaks Zebra which always uses sendto/sendmsg
and creates it's socket without any privileges.

To keep Zebra working don't bother checking if the creator of the
socket has privilege when a destination address is specified.  Instead
rely exclusively on the privileges of the sender of the socket.

Note from Andy: This is exactly Eric's code except for some comment
clarifications and formatting fixes.  Neither I nor, I think, anyone
else is thrilled with this approach, but I'm hesitant to wait on a
better fix since 3.15 is almost here.

Note to stable maintainers: This is a mess.  An earlier series of
patches in 3.15 fix a rather serious security issue (CVE-2014-0181),
but they did so in a way that breaks Zebra.  The offending series
includes:

    commit aa4cf9452f469f16cea8c96283b641b4576d4a7b
    Author: Eric W. Biederman <ebiederm@xmission.com>
    Date:   Wed Apr 23 14:28:03 2014 -0700

        net: Add variants of capable for use on netlink messages

If a given kernel version is missing that series of fixes, it's
probably worth backporting it and this patch.  if that series is
present, then this fix is critical if you care about Zebra.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

This, or something like it, is needed for 3.15.

 include/linux/netlink.h  | 7 ++++---
 net/netlink/af_netlink.c | 7 ++++++-
 2 files changed, 10 insertions(+), 4 deletions(-)

From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 30 May 2014 11:04:00 -0700

> From: "Eric W. Biederman" <ebiederm@xmission.com>
> 
> It was possible to get a setuid root or setcap executable to write to
> it's stdout or stderr (which has been set made a netlink socket) and
> inadvertently reconfigure the networking stack.
> 
> To prevent this we check that both the creator of the socket and
> the currentl applications has permission to reconfigure the network
> stack.
> 
> Unfortunately this breaks Zebra which always uses sendto/sendmsg
> and creates it's socket without any privileges.
> 
> To keep Zebra working don't bother checking if the creator of the
> socket has privilege when a destination address is specified.  Instead
> rely exclusively on the privileges of the sender of the socket.
> 
> Note from Andy: This is exactly Eric's code except for some comment
> clarifications and formatting fixes.  Neither I nor, I think, anyone
> else is thrilled with this approach, but I'm hesitant to wait on a
> better fix since 3.15 is almost here.
> 
> Note to stable maintainers: This is a mess.  An earlier series of
> patches in 3.15 fix a rather serious security issue (CVE-2014-0181),
> but they did so in a way that breaks Zebra.  The offending series
> includes:
> 
>     commit aa4cf9452f469f16cea8c96283b641b4576d4a7b
>     Author: Eric W. Biederman <ebiederm@xmission.com>
>     Date:   Wed Apr 23 14:28:03 2014 -0700
> 
>         net: Add variants of capable for use on netlink messages
> 
> If a given kernel version is missing that series of fixes, it's
> probably worth backporting it and this patch.  if that series is
> present, then this fix is critical if you care about Zebra.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>

Applied, thanks Andy.
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On Fri, 30 May 2014 11:04:00 -0700, Andy Lutomirski wrote:
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -1373,7 +1373,9 @@ retry:
>  bool __netlink_ns_capable(const struct netlink_skb_parms *nsp,
>  			struct user_namespace *user_ns, int cap)
>  {
> -	return sk_ns_capable(nsp->sk, user_ns, cap);
> +	return ((nsp->flags & NETLINK_SKB_DST) ||
> +		file_ns_capable(nsp->sk->sk_socket->file, user_ns, cap)) &&
> +		ns_capable(user_ns, cap);

Sorry if I'm missing something. Is socket->file valid (non-NULL) in
kernel->kernel netlink communication? I don't think it's assigned for
sockets created by netlink_kernel_create, is it? Seems this would cause
NULL ptr dereference.

But then, I don't even know whether kernel->kernel netlink
communication is allowed.

(I'm aware that if this is really the case the NULL ptr deref is not
caused by this patch but by the one this is fixing.)

Thanks,

 Jiri