Re: Linux kernel: CVE-2017-1000112: Exploitable memory corruption due to UFO to non-UFO path switch

[Andrey Konovalov <andreyknvl () gmail com>]

On Thu, Aug 10, 2017 at 10:55 PM, Andrey Konovalov <andreyknvl () gmail com> wrote:
> Hi!
>
> syzkaller found an exploitable memory corruption in UFO code in the
> Linux kernel, the details are below.
>
> ### Bug details
>
> When building a UFO packet with MSG_MORE __ip_append_data() calls
> ip_ufo_append_data() to append. However in between two send() calls,
> the append path can be switched from UFO to non-UFO one, which leads
> to a memory corruption.
>
> In case UFO packet lengths exceeds MTU, copy = maxfraglen - skb->len
> becomes negative on the non-UFO path and the branch to allocate new
> skb is taken. This triggers fragmentation and computation of fraggap =
> skb_prev->len - maxfraglen. Fraggap can exceed MTU, causing copy =
> datalen - transhdrlen - fraggap to become negative. Subsequently
> skb_copy_and_csum_bits() writes out-of-bounds.
>
> A similar issue is present in IPv6 code.
>
> The bug was introduced in e89e9cf539a2 ("[IPv4/IPv6]: UFO
> Scatter-gather approach") on Oct 18 2005.
>
> The fix has been submitted to netdev [1] and should be committed to
> mainline and to stable kernels soon. David has also sent an RFC series
> to remove UFO completely [2], which should be merged in 4.14.
>
> If unprivileged user namespaces are available, this bug can be
> exploited to gain root privileges. I'll share the details and the
> exploit in a few days.
>
> Thanks!
>
> ### Timeline
>
> 2017.08.03 - Bug reported to security () kernel org
> 2017.08.04 - Bug reported to linux-distros@
> 2017.08.10 - Patch submitted to netdev
> 2017.08.10 - Announcement on oss-security@
>
> ### Links
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=85f1bd9a7b5a79d5baa8bf44af19658f7bf77bfa
>
> [2] https://www.spinics.net/lists/netdev/msg443815.html

### Exploitation

The bug can be exploited by an unprivileged user if:

1. User can set up an interface with UFO enabled and MTU < 65535 or
such interface is already present in the system. The former is
possible from inside a user namespace.

2. User can disable the NETIF_F_UFO interface feature or set the
SO_NO_CHECK socket option. The former requires CAP_NET_ADMIN. The
latter is only possible after 40ba330227ad ("udp: disallow UFO for
sockets with SO_NO_CHECK option") from Jan 11 2016. Both are possible
from inside a user namespace.

In particular, the bug can be exploited by an unprivileged user if
unprivileged user namespaces are available.

Below is a link to a proof-of-concept exploit, that gets root on a
range of Ubuntu kernels. The exploit triggers an out-of-bounds write
on a socket buffer and overwrites
skb_shared_info.destructor_arg->callback with a pointer to shellcode.
The exploit includes a SMEP and KASLR bypasses, but no SMAP bypass.

Link: https://github.com/xairy/kernel-exploits/blob/master/CVE-2017-1000112/poc.c