oss-sec mailing list archives
Re: Linux kernel: CVE-2017-6074: DCCP double-free vulnerability (local root)
From: Andrey Konovalov <andreyknvl () google com>
Date: Sun, 26 Feb 2017 01:45:54 +0100
I've uploaded the proof-of-concept exploit here: https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-6074 It includes a SMEP/SMAP bypass, however it's not very reliable. The exploit was tested on Ubuntu 16.04 with 4.4.0-62-generic kernel. It will most likely crash on anything else, unless you at least update the offsets. A little detail that's missing from the initial announcement is that this bug is technically a use-after-free followed by a double-free. The kernel frees skb in dccp_rcv_state_process and then again when destroying the socket due to inet6_destroy_sock. There's actually a lot more stuff going on under the hood, but that's the essential part. The use-after-free happens on skb and skb->data (they are allocated and freed separately though one right after another). Exploiting this would allow us to overwrite skb or skb->data with arbitrary data. The double-free, however, allows us to control what object we overwrite by doing the trick I mentioned in the previous email. To get execution control we can overwrite skb->data, since it has skb_shared_info struct at the end, and shinfo->destructor_arg->callback is a function pointer, which is triggered by skb_release_data. The exploit puts ubuf_info struct and the payload to get root in userspace, so this will be detected by SMAP and SMEP. To disable SMEP and SMAP I used the idea from the CVE-2016-8655 exploit by Philip Pettersson. We can overwrite packet_sock struct, which has a timer_list field deep inside it, which contains a callback and it's argument. We allocate this struct, overwrite the time_list field and schedule the timer. I used native_write_cr4 as the callback and a value with SMEP and SMAP bits disabled for it's argument. Note, that CVE-2016-8655 by itself resulted in a use-after-free on the packet_sock struct, but in this case we make a use-after-free happen by exploiting a double-free. As I mentioned, the exploit is not very reliable, but I don't want to spend any more time on it. The kernel can crash due to a memory corruption if we fail to reallocate some objects in time or in the correct order. However I've managed to make it work on three different environments I have set up (including two vms and a real machine). On Wed, Feb 22, 2017 at 2:28 PM, Andrey Konovalov <andreyknvl () google com> wrote:
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 () kernel 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
Current thread:
- Linux kernel: CVE-2017-6074: DCCP double-free vulnerability (local root) Andrey Konovalov (Feb 22)
- Re: Linux kernel: CVE-2017-6074: DCCP double-free vulnerability (local root) Andrey Konovalov (Feb 26)