Re: [Last-Call] Tsvart last call review of draft-ietf-6man-icmp-limits-07

[Suresh Krishnan <suresh.krishnan@xxxxxxxxx>]

Hi Bernard/Ben,

  Thanks for your review. Just responding to one point below.

On Feb 25, 2020, at 2:34 PM, Benjamin Kaduk <kaduk@xxxxxxx> wrote:

On Tue, Feb 18, 2020 at 01:02:21PM -0800, Bernard Aboba via Datatracker wrote:

Reviewer: Bernard Aboba
Review result: Ready with Issues

TSV-ART Review of draft-ietf-6man-icmp-limits
Bernard Aboba

Result: Ready with Issues

This document specifies several new ICMPv6 errors that can be sent
when a node discards a packet due to it being unable to process the
necessary protocol headers because of processing constraints or
limits.  Reasons include:

     Code (pertinent to this specification)
        1 - Unrecognized Next Header type encountered
        TBA - Extension header too big
        TBA - Extension header chain too long
        TBA - Too many options in extension header
        TBA - Option too big

ICMP Reliability

Section 5.2 states: 

"  ICMP is fundamentally an unreliable protocol and in real deployment
  it may consistently fail over some paths. As with any other use of
  ICMP, it is assumed that the errors defined in this document are only
  best effort to be delivered. No protocol should be implemented that
  relies on reliable delivery of ICMP messages. If necessary,
  alternative or additional mechanisms may used to augment the
  processes used to to deduce the reason that packets are being
  discarded. Such alternative mechanisms are out of scope of this
  specification."

[BA] The last sentence is a bit vague. My assumption is that this is
referring to techniques such as are used in Path MTU discovery (e.g.
tweaking of packets so as to determine potential reasons why packets
are being discarded).  However, a reference might be helpful.

Security Concerns

Pointer field

In Section 3.1, the description of the Pointer field states: 

"      Pointer
        Identifies the octet offset within the invoking packet where
        the problem occurred.

        The pointer will point beyond the end of the ICMPv6 packet if
        the field having a problem is beyond what can fit in the
        maximum size of an ICMPv6 error message."

[BA] I worry about attackers using the Pointer field for
mischief, such as buffer overflows.  The draft currently
does not provide advice to implementers about validating
the Pointer field (e.g. checking it against the length of
the offending packet). Do we really need a 32-bit Pointer field? 

Very reminiscent of heartbleed, even with the note that "The pointer will
point beyond the end of the ICMPv6 packet if the field having a problem is
beyond what can fit in the maximum size of an ICMPv6 error message."

Hmm. This is exactly how base ICMPv6 (RFC4443 and prior to that RFC2463 and RFC1885) defines and uses the Pointer field. And the intent is specifically to be able to point past the end of the packet since the “offending” packet may not be able to fit into the reporting packet. Is there something specific that you think is being enabled by this draft and needs to be addressed?

Regards

Suresh

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