Date: Thu, 10 Aug 2017 22:55:29 +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: Linux kernel: CVE-2017-1000112: Exploitable memory corruption due to UFO to non-UFO path switch 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
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