mtod(m, char *) + len
is wrong, an option is allowed to be located in another mbuf of the chain.
If the offset of an option within the chain is bigger than the length of
the first mbuf in that chain, we are reading/writing one byte of packet-
controlled data beyond the end of the first mbuf.
The length of this first mbuf depends on the layout the network driver
chose. In the most difficult case, it will allocate a 2KB cluster, which
is bigger than the Ethernet MTU.
But there is at least one way of exploiting this case: by sending a
special combination of nested IPv6 fragments, the packet can control a
good bunch of 'len'. By luck, the memory pool containing clusters does not
embed the pool header in front of the items, so it is not straightforward
to predict what is located at 'mtod(m, char *) + len'.
However, by sending offending fragments in a loop, it is possible to
crash the kernel - at some point we will hit important data structures.
As far as I can tell, PF protects against this difficult case, because
it kicks nested fragments. NPF does not protect against this. IPF I don't
know.
Then there are the more easy cases, if the MTU is bigger than a cluster,
or if the network driver did not allocate a cluster, or perhaps if the
fragments are received via a tunnel; I haven't investigated these cases.
Change ip6_get_prevhdr so that it returns an offset in the chain, and
always use IP6_EXTHDR_GET to get a writable pointer. IP6_EXTHDR_GET
leaves M_PKTHDR untouched.
This place is still fragile.
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