Re: Proposal for missing blob support in Git repos

[Jonathan Tan <jonathantanmy@google.com>]

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? If yes, I think that is the nature of executing an arbitrary 
command. If we really want to avoid that, we could drop the hook 
functionality (and instead, for example, provide the URL of a Git repo 
instead from which we can communicate using a new fetch-blob protocol), 
although that would reduce the usefulness of this, especially during the 
transition period in which we don't have any sort of batching of requests.