Re: [PATCH 0/7] PREVIEW: Introduce DC_AND_OPENSSL_SHA1 make flag

[Jeff King <peff@peff.net>]

On Fri, Mar 24, 2017 at 11:37:54PM -0700, Junio C Hamano wrote:

> The hash that names a packfile is constructed by sorting all the
> names of the objects contained in the packfile and running SHA-1
> hash over it.  I think this MUST be hashed with collision-attack
> detection.  A malicious site can feed you a packfile that contains
> objects the site crafts so that the sorted object names would result
> in a collision-attack, ending up with one pack that contains a sets
> of objects different from another pack that happens to have the same
> packname, causing Git to say "Ah, this new pack must have the same
> set of objects as the pack we already have" and discard it,
> resulting in lost objects and a corrupt repository with missing
> objects.

I don't think this case really matters for collision detection. What's
important is what Git does when it receives a brand-new packfile that
would overwrite an existing one. It _should_ keep the old one, under the
usual "existing data wins" rule.

It should be easy to test, though:

  $ git init tmp && cd tmp
  $ git commit --allow-empty -m foo
  $ git gc

  $ touch -d yesterday .git/objects/pack/*
  $ ls -l .git/objects/pack
  -r--r--r-- 1 peff peff 1128 Mar 25 02:10 pack-7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b.idx
  -r--r--r-- 1 peff peff  153 Mar 25 02:10 pack-7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b.pack

  $ git index-pack --stdin <.git/objects/pack/*.pack
  pack	7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b

  $ ls -l .git/objects/pack
  -r--r--r-- 1 peff peff 1128 Mar 25 02:10 pack-7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b.idx
  -r--r--r-- 1 peff peff  153 Mar 25 02:10 pack-7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b.pack

Looks like the timestamps were retained. <phew> And if we use strace, we
can see what happens:

  $ strace git index-pac k--stdin <.git/objects/pack/*.pack
  link(".git/objects/pack/tmp_pack_YSrdWU", ".git/objects/pack/pack-7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b.pack") = -1 EEXIST (File exists)
  unlink(".git/objects/pack/tmp_pack_YSrdWU") = 0
  link(".git/objects/pack/tmp_idx_O94NNU", ".git/objects/pack/pack-7e9d64ac27adc9ce1b12774dd287ff9bd8a9345b.idx") = -1 EEXIST (File exists)
  unlink(".git/objects/pack/tmp_idx_O94NNU") = 0

This is due to the link()/EEXIST handling in finalize_object_file. It
has a FIXME for a collision check, so we could actually detect at that
point whether we have a real collision, or if the other side just
happened to send us the same pack.

I wouldn't be surprised if the dumb-http walker is not so careful,
though (but the solution is to make it careful, not to worry about
a weak hash algorithm).

The rest of your email all made sense to me.

-Peff