Date: Sun, 26 Feb 2017 01:45:54 +0100 From: Andrey Konovalov <andreyknvl@...gle.com> To: oss-security@...ts.openwall.com Subject: Re: Linux kernel: CVE-2017-6074: DCCP double-free vulnerability (local root) 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@...gle.com> wrote: > Hi, > > This is an announcement about CVE-2017-6074  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 . > > ### 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 . > > 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 , 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 > >  http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=2017-6074 >  https://github.com/google/syzkaller >  http://lxr.free-electrons.com/source/net/dccp/input.c?v=4.9#L606 >  http://lxr.free-electrons.com/source/net/dccp/ipv6.c?v=4.9#L351 >  https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5edabca9d4cff7f1f2b68f0bac55ef99d9798ba4
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