netdev - Re: Netlink mmap tx security?

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Date:	Mon, 12 May 2014 14:08:20 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"David S. Miller" <davem@...emloft.net>,
	Network Development <netdev@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: Netlink mmap tx security?

[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]

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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