git/send-pack.c, branch v2.2.1

Merge branch 'jc/push-cert'

2014-10-08T20:05:25Z

Allow "git push" request to be signed, so that it can be verified and audited, using the GPG signature of the person who pushed, that the tips of branches at a public repository really point the commits the pusher wanted to, without having to "trust" the server. * jc/push-cert: (24 commits) receive-pack::hmac_sha1(): copy the entire SHA-1 hash out signed push: allow stale nonce in stateless mode signed push: teach smart-HTTP to pass "git push --signed" around signed push: fortify against replay attacks signed push: add "pushee" header to push certificate signed push: remove duplicated protocol info send-pack: send feature request on push-cert packet receive-pack: GPG-validate push certificates push: the beginning of "git push --signed" pack-protocol doc: typofix for PKT-LINE gpg-interface: move parse_signature() to where it should be gpg-interface: move parse_gpg_output() to where it should be send-pack: clarify that cmds_sent is a boolean send-pack: refactor inspecting and resetting status and sending commands send-pack: rename "new_refs" to "need_pack_data" receive-pack: factor out capability string generation send-pack: factor out capability string generation send-pack: always send capabilities send-pack: refactor decision to send update per ref send-pack: move REF_STATUS_REJECT_NODELETE logic a bit higher ...

signed push: fortify against replay attacks

2014-09-17T21:27:40Z

In order to prevent a valid push certificate for pushing into an repository from getting replayed in a different push operation, send a nonce string from the receive-pack process and have the signer include it in the push certificate. The receiving end uses an HMAC hash of the path to the repository it serves and the current time stamp, hashed with a secret seed (the secret seed does not have to be per-repository but can be defined in /etc/gitconfig) to generate the nonce, in order to ensure that a random third party cannot forge a nonce that looks like it originated from it. The original nonce is exported as GIT_PUSH_CERT_NONCE for the hooks to examine and match against the value on the "nonce" header in the certificate to notice a replay, but returned "nonce" header in the push certificate is examined by receive-pack and the result is exported as GIT_PUSH_CERT_NONCE_STATUS, whose value would be "OK" if the nonce recorded in the certificate matches what we expect, so that the hooks can more easily check. Signed-off-by: Junio C Hamano

signed push: add "pushee" header to push certificate

2014-09-15T20:23:28Z

Record the URL of the intended recipient for a push (after anonymizing it if it has authentication material) on a new "pushee URL" header. Because the networking configuration (SSH-tunnels, proxies, etc.) on the pushing user's side varies, the receiving repository may not know the single canonical URL all the pushing users would refer it as (besides, many sites allow pushing over ssh://host/path and https://host/path protocols to the same repository but with different local part of the path). So this value may not be reliably used for replay-attack prevention purposes, but this will still serve as a human readable hint to identify the repository the certificate refers to. Signed-off-by: Junio C Hamano

signed push: remove duplicated protocol info

2014-09-15T20:23:28Z

With the interim protocol, we used to send the update commands even though we already send a signed copy of the same information when push certificate is in use. Update the send-pack/receive-pack pair not to do so. The notable thing on the receive-pack side is that it makes sure that there is no command sent over the traditional protocol packet outside the push certificate. Otherwise a pusher can claim to be pushing one set of ref updates in the signed certificate while issuing commands to update unrelated refs, and such an update will evade later audits. Finally, start documenting the protocol. Signed-off-by: Junio C Hamano

send-pack: send feature request on push-cert packet

2014-09-15T20:23:28Z

We would want to update the interim protocol so that we do not send the usual update commands when the push certificate feature is in use, as the same information is in the certificate. Once that happens, the push-cert packet may become the only protocol command, but then there is no packet to put the feature request behind, like we always did. As we have prepared the receiving end that understands the push-cert feature to accept the feature request on the first protocol packet (other than "shallow ", which was an unfortunate historical mistake that has to come before everything else), we can give the feature request on the push-cert packet instead of the first update protocol packet, in preparation for the next step to actually update to the final protocol. Signed-off-by: Junio C Hamano

push: the beginning of "git push --signed"

2014-09-15T20:23:20Z

While signed tags and commits assert that the objects thusly signed came from you, who signed these objects, there is not a good way to assert that you wanted to have a particular object at the tip of a particular branch. My signing v2.0.1 tag only means I want to call the version v2.0.1, and it does not mean I want to push it out to my 'master' branch---it is likely that I only want it in 'maint', so the signature on the object alone is insufficient. The only assurance to you that 'maint' points at what I wanted to place there comes from your trust on the hosting site and my authentication with it, which cannot easily audited later. Introduce a mechanism that allows you to sign a "push certificate" (for the lack of better name) every time you push, asserting that what object you are pushing to update which ref that used to point at what other object. Think of it as a cryptographic protection for ref updates, similar to signed tags/commits but working on an orthogonal axis. The basic flow based on this mechanism goes like this: 1. You push out your work with "git push --signed". 2. The sending side learns where the remote refs are as usual, together with what protocol extension the receiving end supports. If the receiving end does not advertise the protocol extension "push-cert", an attempt to "git push --signed" fails. Otherwise, a text file, that looks like the following, is prepared in core: certificate version 0.1 pusher Junio C Hamano 1315427886 -0700 7339ca65... 21580ecb... refs/heads/master 3793ac56... 12850bec... refs/heads/next The file begins with a few header lines, which may grow as we gain more experience. The 'pusher' header records the name of the signer (the value of user.signingkey configuration variable, falling back to GIT_COMMITTER_{NAME|EMAIL}) and the time of the certificate generation. After the header, a blank line follows, followed by a copy of the protocol message lines. Each line shows the old and the new object name at the tip of the ref this push tries to update, in the way identical to how the underlying "git push" protocol exchange tells the ref updates to the receiving end (by recording the "old" object name, the push certificate also protects against replaying). It is expected that new command packet types other than the old-new-refname kind will be included in push certificate in the same way as would appear in the plain vanilla command packets in unsigned pushes. The user then is asked to sign this push certificate using GPG, formatted in a way similar to how signed tag objects are signed, and the result is sent to the other side (i.e. receive-pack). In the protocol exchange, this step comes immediately before the sender tells what the result of the push should be, which in turn comes before it sends the pack data. 3. When the receiving end sees a push certificate, the certificate is written out as a blob. The pre-receive hook can learn about the certificate by checking GIT_PUSH_CERT environment variable, which, if present, tells the object name of this blob, and make the decision to allow or reject this push. Additionally, the post-receive hook can also look at the certificate, which may be a good place to log all the received certificates for later audits. Because a push certificate carry the same information as the usual command packets in the protocol exchange, we can omit the latter when a push certificate is in use and reduce the protocol overhead. This however is not included in this patch to make it easier to review (in other words, the series at this step should never be released without the remainder of the series, as it implements an interim protocol that will be incompatible with the final one). As such, the documentation update for the protocol is left out of this step. Signed-off-by: Junio C Hamano

send-pack: clarify that cmds_sent is a boolean

2014-09-15T20:23:20Z

We use it to make sure that the feature request is sent only once on the very first request packet (ignoring the "shallow " line, which was an unfortunate mistake we cannot retroactively fix with existing receive-pack already deployed in the field) and we set it to "true" with cmds_sent++, not because we care about the actual number of updates sent but because it is merely an idiomatic way. Set it explicitly to one to clarify that the code that uses this variable only cares about its zero-ness. Signed-off-by: Junio C Hamano

send-pack: refactor inspecting and resetting status and sending commands

2014-09-15T20:23:19Z

The main loop over remote_refs list inspects the ref status to see if we need to generate pack data (i.e. a delete-only push does not need to send any additional data), resets it to "expecting the status report" state, and formats the actual update commands to be sent. Split the former two out of the main loop, as it will become conditional in later steps. Besides, we should have code that does real thing here, before the "Finally, tell the other end!" part ;-) Signed-off-by: Junio C Hamano

send-pack: rename "new_refs" to "need_pack_data"

2014-09-15T20:23:19Z

The variable counts how many non-deleting command is being sent, but is only checked with 0-ness to decide if we need to send the pack data. Signed-off-by: Junio C Hamano

send-pack: factor out capability string generation

2014-09-15T20:23:19Z

A run of 'var ? " var" : ""' fed to a long printf string in a deeply nested block was hard to read. Move it outside the loop and format it into a strbuf. As an added bonus, the trick to add "agent=" by using two conditionals is replaced by a more readable version. Signed-off-by: Junio C Hamano