[APPLETALK]: Fix a remotely triggerable crash

When we receive an AppleTalk frame shorter than what its header says,
we still attempt to verify its checksum, and trip on the BUG_ON() at
the end of function atalk_sum_skb() because of the length mismatch.

This has security implications because this can be triggered by simply
sending a specially crafted ethernet frame to a target victim,
effectively crashing that host. Thus this qualifies, I think, as a
remote DoS. Here is the frame I used to trigger the crash, in npg
format:

<Appletalk Killer>
{
# Ethernet header -----

XX XX XX XX XX XX # Destination MAC
00 00 00 00 00 00 # Source MAC
00 1D # Length

# LLC header -----

AA AA 03
08 00 07 80 9B # Appletalk

# Appletalk header -----

00 1B # Packet length (invalid)
00 01 # Fake checksum
00 00 00 00 # Destination and source networks
00 00 00 00 # Destination and source nodes and ports

# Payload -----

0C 0D 0E 0F 10 11 12 13
14
}

The destination MAC address must be set to those of the victim.

The severity is mitigated by two requirements:
* The target host must have the appletalk kernel module loaded. I
suspect this isn't so frequent.
* AppleTalk frames are non-IP, thus I guess they can only travel on
local networks. I am no network expert though, maybe it is possible
to somehow encapsulate AppleTalk packets over IP.

The bug has been reported back in June 2004:
http://bugzilla.kernel.org/show_bug.cgi?id=2979
But it wasn't investigated, and was closed in July 2006 as both
reporters had vanished meanwhile.

This code was new in kernel 2.6.0-test5:
http://git.kernel.org/?p=linux/kernel/git/tglx/history.git;a=commitdiff;h=7ab442d7e0a76402c12553ee256f756097cae2d2
And not modified since then, so we can assume that vanilla kernels
2.6.0-test5 and later, and distribution kernels based thereon, are
affected.

Note that I still do not know for sure what triggered the bug in the
real-world cases. The frame could have been corrupted by the kernel if
we have a bug hiding somewhere. But more likely, we are receiving the
faulty frame from the network.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>