Re: Proposal for missing blob support in Git repos

From: Junio C Hamano <gitster@pobox.com>
Date: May 2, 2017, 01:41 UTC
Message-ID: <xmqq37cof320.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <c0c8a0c3-582c-cf3b-3833-c918a0630f9f@google.com>

Jonathan Tan <jonathantanmy@google.com> writes:

Show 19 quoted lines

> On 05/01/2017 04:29 PM, Junio C Hamano wrote:
>> Jonathan Tan <jonathantanmy@google.com> writes:
>>
>>> Thanks for your comments. If you're referring to the codepath
>>> involving write_sha1_file() (for example, builtin/hash-object ->
>>> index_fd or builtin/unpack-objects), that is fine because
>>> write_sha1_file() invokes freshen_packed_object() and
>>> freshen_loose_object() directly to check if the object already exists
>>> (and thus does not invoke the new mechanism in this patch).
>>
>> Is that a good thing, though?  It means that you an attacker can
>> feed one version to the remote object store your "grab blob" hook
>> gets the blobs from, and have you add a colliding object locally,
>> and the usual "are we recording the same object as existing one?"
>> check is bypassed.
>
> If I understand this correctly, what you mean is the situation where
> the hook adds an object to the local repo, overriding another object
> of the same name?

No.

write_sha1_file() pays attention to objects already in the local object store to avoid hash collisions that can be used to replace a known-to-be-good object and that is done as a security measure. What I am reading in your response was that this new mechanism bypasses that, and I was wondering if that is a good thing.