Date: Sun, 13 Aug 2017 18:21:55 +0200 From: Andrey Konovalov <andreyknvl@...il.com> To: oss-security@...ts.openwall.com Cc: willemdebruijn.kernel@...il.com, Dmitry Vyukov <dvyukov@...gle.com>, Kostya Serebryany <kcc@...gle.com> Subject: Re: Linux kernel: CVE-2017-1000112: Exploitable memory corruption due to UFO to non-UFO path switch On Thu, Aug 10, 2017 at 10:55 PM, Andrey Konovalov <andreyknvl@...il.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  and should be committed to > mainline and to stable kernels soon. David has also sent an RFC series > to remove UFO completely , 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@...nel.org > 2017.08.04 - Bug reported to linux-distros@ > 2017.08.10 - Patch submitted to netdev > 2017.08.10 - Announcement on oss-security@ > > ### Links > >  https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=85f1bd9a7b5a79d5baa8bf44af19658f7bf77bfa > >  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
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