netdev - Re: Netlink mmap tx security?

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Date:	Sat, 11 Oct 2014 15:29:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: Netlink mmap tx security?

On May 12, 2014 3:08 PM, "Andy Lutomirski" <luto@...capital.net> wrote:
>
> [moving to netdev -- this is much lower impact than I thought, since
> you can't set up a netlink mmap ring without global CAP_NET_ADMIN]

Did anything ever happen here?  Despite not being a privilege
escalation in the normal sense, it's still a bug, and it's still a
fairly easy bypass of module signatures.


--Andy

>
> On Wed, May 7, 2014 at 5:46 PM, Andy Lutomirski <luto@...capital.net> wrote:
> > Disclaimer: I haven't tried to write a proof-of-concept, so I could be
> > wrong here.
> >
> >     /* Netlink messages are validated by the receiver before processing.
> >      * In order to avoid userspace changing the contents of the message
> >      * after validation, the socket and the ring may only be used by a
> >      * single process, otherwise we fall back to copying.
> >      */
> >     if (atomic_long_read(&sk->sk_socket->file->f_count) > 2 ||
> >         atomic_read(&nlk->mapped) > 1)
> >         excl = false;
> >
> > How is this possibly safe?  I think it's broken for at least three reasons:
> >
> > 1. Shouldn't that be atomic_long(read(&f_count) > 1))?
> >
> > 2. threads
> >
> > 3. process_vm_writev
> >
> > I wouldn't be surprised if RDMA and AIO also break this assumption.
> >
> > Does anything rely on mmapped netlink tx being fast?  If not, can this
> > code just be deleted?
> >
> > For that matter, does anything rely on mmapped netlink tx at all?
> >
> > (On a non-KASLR, non-SMEP system, this probably gives reliable kernel
> > code execution.)
> >
> > For that matter, netlink_mmap_sendmsg also appears to vulnerable to a
> > TOCTOU attack via hdr.
> >
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