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

Benjamin Kaduk <kaduk@mit.edu> Thu, 27 February 2020 06:33 UTC

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Date: Wed, 26 Feb 2020 22:33:45 -0800
From: Benjamin Kaduk <kaduk@mit.edu>
To: Suresh Krishnan <suresh.krishnan@gmail.com>
Cc: Bernard Aboba <bernard.aboba@gmail.com>, tsv-art@ietf.org, draft-ietf-6man-icmp-limits.all@ietf.org, 6man <ipv6@ietf.org>, last-call@ietf.org
Subject: Re: [Last-Call] Tsvart last call review of draft-ietf-6man-icmp-limits-07
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Hi Suresh,

On Tue, Feb 25, 2020 at 11:01:53PM -0500, Suresh Krishnan wrote:
> Hi Bernard/Ben,
>   Thanks for your review. Just responding to one point below.
> 
> > On Feb 25, 2020, at 2:34 PM, Benjamin Kaduk <kaduk@MIT.EDU> 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?

I don't want to steal Bernard's thunder, but I'd consider a note along the
lines of "in such cases the pointer cannot be dereferenced without the
original packet" (noting that the statement in quotation marks is not,
strictly speaking, true, so further edits would be needed).

-Ben