Linux kernel: CVE-2017-1000112: Exploitable memory corruption due to UFO to non-UFO path switch

[Andrey Konovalov <andreyknvl () gmail com>]

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 [1] and should be committed to
mainline and to stable kernels soon. David has also sent an RFC series
to remove UFO completely [2], 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 () kernel org
2017.08.04 - Bug reported to linux-distros@
2017.08.10 - Patch submitted to netdev
2017.08.10 - Announcement on oss-security@

### Links

[1] https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=85f1bd9a7b5a79d5baa8bf44af19658f7bf77bfa

[2] https://www.spinics.net/lists/netdev/msg443815.html