Verify the packet length in sctp6_input().

The sctp6_ctlinput() function does not properly check the length of the packet
it receives from the ICMP6 input routine. This means that an attacker can craft
a packet that will cause a kernel panic.

When the kernel receives an ICMP6 error message with one of the types/codes
it handles, it calls icmp6_notify_error() to deliver it to the upper-level
protocol. icmp6_notify_error() cycles through the extension headers (if any)
to find the protocol number of the first non-extension header. It does NOT
verify the length of the non-extension header.

It passes information about the packet (including the actual packet) to the
upper-level protocol's pr_ctlinput function. In the case of SCTP for IPv6,
icmp6_notify_error() calls sctp6_ctlinput().

sctp6_ctlinput() assumes that the incoming packet contains a sufficiently-long
SCTP header and calls m_copydata() to extract a copy of that header. In turn,
m_copydata() assumes that the caller has already verified that the offset and
length parameters are correct. If they are incorrect, it will dereference a
NULL pointer and cause a kernel panic.

In short, no one is sufficiently verifying the input, and the result is a
kernel panic.

Submitted by:	jtl
Security:	SA-16:01.sctp

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