Re: [gentoo-dev] [RFC] Anti-spam for goose

[Fabian Groffen]

Hi,

On 21-05-2020 10:47:07 +0200, Michał Górny wrote:
> Hi,
>
> TL;DR: I'm looking for opinions on how to protect goose from spam,
> i.e. mass fake submissions.
>
>
> Problem
> =======
> Goose currently lacks proper limiting of submitted data.  The only
> limiter currently in place is based on unique submitter id that is
> randomly generated at setup time and in full control of the submitter.
> This only protects against accidental duplicates but it can't protect
> against deliberate action.
>
> An attacker could easily submit thousands (millions?) of fake entries by
> issuing a lot of requests with different ids.  Creating them is
> as trivial as using successive numbers.  The potential damage includes:

Perhaps you could consider something like a reputation system.  I'm
thinking of things like only publishing results after X hours when an id
is new (graylisting?), and gradually build up "trust" there.  In the X
hours you could then determine something is potentially fraud if you see
a new user id, loads of submissions from same IP, etc. what you describe
below I think.

The reputation logic could further build on if it appears to follow a
norm, e.g. compilation times which fall in the average given the
cpu/arch configuration.
Another way would be to see submissions for packages that are actually
bumped/stabilised in the tree, to score an id as more likely to be
genuine.

I think it will be a tad complicated, but static limiting might be as
easy to circumvent as you block it, as has been pointed out already.

Perhaps, it is fruitful to think of the reverse, when is something
obviously bad?  When a single (obscure?) package is suddenly reported
many times by new ids?  When a single id generates hundreds or thousands
of package submissions (is it a cluster being misconfigured, many
identical packages, or what seems to be an a to z scan).
Thing is, would a single "fake" submission (that IMO will unlikely be
ever noticed) screw up the overall state of things?  I think the
fuzzyness of the system as a whole should cover for these.  It is pure
poisioning that should be able to be mitigated, and I agree with you
preferably most of it blocked by default.  Fact probably is that it will
happen nevertheless.

That brings me to the thought: are there things that can be done to make
sure a fraudulous action can be easily undone or negated somehow?  E.g.
should a log be kept, or some action to rollback and replay.  Sorry to
have no concrete examples here.

Fabian

>
> - distorting the metrics to the point of it being useless (even though
> some people consider it useless by design).
>
> - submitting lots of arbitrary data to cause DoS via growing
> the database until no disk space is left.
>
> - blocking large range of valid user ids, causing collisions with
> legitimate users more likely.
>
> I don't think it worthwhile to discuss the motivation for doing so:
> whether it would be someone wishing harm to Gentoo, disagreeing with
> the project or merely wanting to try and see if it would work.  The case
> of SKS keyservers teaches us a lesson that you can't leave holes like
> this open a long time because someone eventually will abuse them.
>
>
> Option 1: IP-based limiting
> ===========================
> The original idea was to set a hard limit of submissions per week based
> on IP address of the submitter.  This has (at least as far as IPv4 is
> concerned) the advantages that:
>
> - submitted has limited control of his IP address (i.e. he can't just
> submit stuff using arbitrary data)
>
> - IP address range is naturally limited
>
> - IP addresses have non-zero cost
>
> This method could strongly reduce the number of fake submissions one
> attacker could devise.  However, it has a few problems too:
>
> - a low limit would harm legitimate submitters sharing IP address
> (i.e. behind NAT)
>
> - it actively favors people with access to large number of IP addresses
>
> - it doesn't map cleanly to IPv6 (where some people may have just one IP
> address, and others may have whole /64 or /48 ranges)
>
> - it may cause problems for anonymizing network users (and we want to
> encourage Tor usage for privacy)
>
> All this considered, IP address limiting can't be used the primary
> method of preventing fake submissions.  However, I suppose it could work
> as an additional DoS prevention, limiting the number of submissions from
> a single address over short periods of time.
>
> Example: if we limit to 10 requests an hour, then a single IP can be
> used ot manufacture at most 240 submissions a day.  This might be
> sufficient to render them unusable but should keep the database
> reasonably safe.
>
>
> Option 2: proof-of-work
> =======================
> An alternative of using a proof-of-work algorithm was suggested to me
> yesterday.  The idea is that every submission has to be accompanied with
> the result of some cumbersome calculation that can't be trivially run
> in parallel or optimized out to dedicated hardware.
>
> On the plus side, it would rely more on actual physical hardware than IP
> addresses provided by ISPs.  While it would be a waste of CPU time
> and memory, doing it just once a week wouldn't be that much harm.
>
> On the minus side, it would penalize people with weak hardware.
>
> For example, 'time hashcash -m -b 28 -r test' gives:
>
> - 34 s (-s estimated 38 s) on Ryzen 5 3600
>
> - 3 minutes (estimated 92 s) on some old 32-bit Celeron M
>
> At the same time, it would still permit a lot of fake submissions.  For
> example, randomx [1] claims to require 2G of memory in fast mode.  This
> would still allow me to use 7 threads.  If we adjusted the algorithm to
> take ~30 seconds, that means 7 submissions every 30 s, i.e. 20k
> submissions a day.
>
> So in the end, while this is interesting, it doesn't seem like
> a workable anti-spam measure.
>
>
> Option 3: explicit CAPTCHA
> ==========================
> A traditional way of dealing with spam -- require every new system
> identifier to be confirmed by solving a CAPTCHA (or a few identifiers
> for one CAPTCHA).
>
> The advantage of this method is that it requires a real human work to be
> performed, effectively limiting the ability to submit spam.
> The disadvantage is that it is cumbersome to users, so many of them will
> just resign from participating.
>
>
> Other ideas
> ===========
> Do you have any other ideas on how we could resolve this?
>
>
> [1] https://github.com/tevador/RandomX
>
>
> --
> Best regards,
> Michał Górny
>



-- 
Fabian Groffen
Gentoo on a different level