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Date: Wed, 22 Feb 2017 14:28:35 +0100
From: Andrey Konovalov <andreyknvl@...gle.com>
To: oss-security@...ts.openwall.com
Subject: Linux kernel: CVE-2017-6074: DCCP double-free vulnerability (local root)

Hi,

This is an announcement about CVE-2017-6074 [1] which is a double-free
vulnerability I found in the Linux kernel. It can be exploited to gain
kernel code execution from an unprivileged processes.

Fixed on Feb 17, 2017:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5edabca9d4cff7f1f2b68f0bac55ef99d9798ba4

The oldest version that was checked is 2.6.18 (Sep 2006), which is
vulnerable. However, the bug was introduced before that, probably in
the first release with DCCP support (2.6.14, Oct 2005).

The kernel needs to be built with CONFIG_IP_DCCP for the vulnerability
to be present. A lot of modern distributions enable this option by
default.

The bug was found with syzkaller [2].

### Bug details

In the current DCCP implementation an skb for a DCCP_PKT_REQUEST
packet is forcibly freed via __kfree_skb in dccp_rcv_state_process if
dccp_v6_conn_request successfully returns [3].

However, if IPV6_RECVPKTINFO is set on a socket, the address of the
skb is saved to ireq->pktopts and the ref count for skb is incremented
in dccp_v6_conn_request [4], so skb is still in use. Nevertheless, it
still gets freed in dccp_rcv_state_process.

The fix is to call consume_skb, which accounts for skb->users,
instead of doing goto discard and therefore calling __kfree_skb.

To exploit this double-free, it can be turned into a use-after-free:

//  The first free:
kfree(dccp_skb)
// Another object allocated on the same place as dccp_skb:
some_object = kmalloc()
// The second free, effectively frees some_object
kfree(dccp_skb)

As this point we have a use-after-free on some_object. An attacker can
control what object that would be and overwrite it's content with
arbitrary data by using some of the kernel heap spraying techniques.
If the overwritten object has any triggerable function pointers, an
attacker gets to execute arbitrary code within the kernel.

I'll publish an exploit in a few days, giving people time to update.

New Ubuntu kernels are out so please update as soon as possible.

### Timeline

2017-02-15: Bug reported to security@...nel.org
2017-02-16: Patch submitted to netdev
2017-02-17: Patch committed to mainline kernel
2017-02-18: Notification sent to linux-distros
2017-02-22: Public announcement

### Links

[1] http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=2017-6074
[2] https://github.com/google/syzkaller
[3] http://lxr.free-electrons.com/source/net/dccp/input.c?v=4.9#L606
[4] http://lxr.free-electrons.com/source/net/dccp/ipv6.c?v=4.9#L351
[5] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5edabca9d4cff7f1f2b68f0bac55ef99d9798ba4

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