Re: git-clone causes out of memory

[Derrick Stolee <stolee@gmail.com>]

On 10/13/2017 8:44 AM, Jeff King wrote:
> On Fri, Oct 13, 2017 at 03:12:43PM +0300, Constantine wrote:
>
>> On 13.10.2017 15:04, Junio C Hamano wrote:
>>> Christian Couder <christian.couder@gmail.com> writes:
>>>
>>>> Yeah, but perhaps Git could be smarter when rev-listing too and avoid
>>>> processing files or directories it has already seen?
>>> Aren't you suggesting to optimize for a wrong case?
>>>
>> Anything that is possible with a software should be considered as a possible
>> usecase. It's in fact a DoS attack. Imagine there's a server that using git
>> to process something, and now there's a way to knock down this server. It's
>> also bad from a promotional stand point.
> But the point is that you'd have the same problem with any repository
> that had 10^7 files in it. Yes, it's convenient for the attacker that
> there are only 9 objects, but fundamentally it's pretty easy for an
> attacker to construct repositories that have large trees (or very deep
> trees -- that's what causes stack exhaustion in some cases).
>
> Note too that this attack almost always comes down to the diff code
> (which is why it kicks in for pathspec limiting) which has to actually
> expand the tree. Most "normal" server-side operations (like accepting
> pushes or serving fetches) operate only on the object graph and _do_
> avoid processing already-seen objects.
>
> As soon as servers start trying to checkout or diff, though, the attack
> surface gets quite large. And you really need to start thinking about
> having resource limits and quotas for CPU and memory use of each process
> (and group by requesting user, IP, repo, etc).
>
> I think the main thing Git could be doing here is to limit the size of
> the tree (both width and depth). But arbitrary limits like that have a
> way of being annoying, and I think it just pushes resource-exhaustion
> attacks off a little (e.g., can you construct a blob that behaves badly
> with the "--patch"?).
>
> -Peff

I'm particularly interested in why `git rev-list HEAD -- [path]` gets 
slower in this case, because I wrote the history algorithm used by VSTS. 
In our algorithm, we only walk the list of objects from commit to the 
tree containing the path item. For example, in the path d0/d0/d0, we 
would only walk:

     commit --root--> tree --d0--> tree --d0--> tree [parse oid for d0 
entry]

 From this, we can determine the TREESAME relationship by parsing four 
objects without parsing all contents below d0/d0/d0.

The reason we have exponential behavior in `git rev-list` is because we 
are calling diff_tree_oid() in tree-diff.c recursively without 
short-circuiting on equal OIDs.

I will prepare a patch that adds this OID-equal short-circuit to avoid 
this exponential behavior. I'll model my patch against a similar patch 
in master:

     commit d12a8cf0af18804c2000efc7a0224da631e04cd1 unpack-trees: avoid 
duplicate ODB lookups during checkout

It will also significantly speed up rev-list calls for short paths in 
deep repositories. It will not be very measurable in the git or Linux 
repos because their shallow folder structure.

Thanks,
-Stolee