Re: [PATCH v3 0/8] refs: add symref support to 'git-update-ref'

From: Karthik Nayak <karthik.188@gmail.com>
Date: Apr 24, 2024, 16:25 UTC
Message-ID: <CAOLa=ZRk8QR4qkbkRm6AirapdrAqz6bG-tXfY3zRQU_9XkJM1Q@mail.gmail.com>
In-Reply-To: <20240423220308.GC1172807@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 45 quoted lines

> On Tue, Apr 23, 2024 at 11:28:10PM +0200, Karthik Nayak wrote:
>
>> Changes from v2:
>> 1. We no longer have separate commands for symrefs, instead the regular
>> commands learn to parse 'ref:<target>' as symref targets. This reduces
>> the code in this series. Thanks Patrick for the suggestion.
>
> Hmm. I can see how this makes things a lot simpler, but it introduces an
> ambiguity, since you can pass full ref expressions to "update-ref" (like
> "ref:foo" to find the "foo" entry in ref^{tree}). I see that you only
> kick in the symref "ref:" handling if the regular oid lookup failed, so
> there's no backwards-compatibility issue (anything that used to work
> will still take precedence, and the new code only runs when the old code
> would have reported an error).
>
> But I wonder if it would let somebody cause mischief in a repository
> they can push to, but which may get updates from other sources. For
> example, imagine a forge like GitLab runs the equivalent of:
>
>   echo "create refs/whatever ref:refs/heads/main" |
>   git update-ref --stdin
>
> as part of some system process. Now if I push up "refs/heads/ref" that
> contains the path "refs/heads/main" in its tree, that will take
> precedence, causing the system process to do something it did not
> expect.
>
> I think you'd have to pile on a lot of assumptions to get any kind of
> security problem. Something like:
>
>  1. The system has a hidden ref namespace like refs/gitlab that normal
>     remote push/fetch users are not allowed to read/write to.
>
>  2. The system tries to make a symlink within that namespace. Say,
>     "refs/gitlab/metadata/HEAD" to point to
>     "refs/gitlab/metadata/branches/main" or something.
>
>  3. The user pushes up "refs/heads/ref" with a tree that contains
>     "refs/gitlab/metadata/branches/main". Now when (2) happens, the
>     hidden ref points to user-controlled data.
>
> That's pretty convoluted. But we can avoid it entirely if there's no
> ambiguity in the protocol at all.
>
> -Peff

Thanks Peff, that is indeed something I totally missed thinking about.

This also brings light onto the previous versions we were considering:

    symref-update SP <ref> SP <new-target> [SP (<old-target> | <old-oid>)] LF

There is also some ambiguity here which we missed, especially when we support dangling refs. If we're updating a dangling ref <ref>, and we provide an old value. Then there is uncertainty around whether the provided value is actually a <old-target> or if it's an <old-oid>.

For non dangling ref symref, we first parse it as an <old-target> and since the <old-target> would exist, we can move on.

So I see two ways to go about this,

1. In the symref-update function, we need to parse and see if <ref> is a
regular ref or a symref, if it is symref, we simply set the provided old
value as <old-target>, if not, we set it as <old-oid>. This seems clunky
because we'll be parsing the ref and trying to understand its type in
'update-ref.c', before the actual update.

2. We change the syntax to something like

    symref-update SP <ref> SP <new-ref> [SP (ref <old-target> | oid
<old-oid>)] LF

this would remove any ambiguity since the user specifies the data type they're providing.

Overall, I think it is best to discard this version and I will push v4 with the older schematics of introducing new commands.

I'm currently considering going ahead with the [2], but will wait for a day or two to consider other opinions.

Also on a sidenote, it's worth considering that with the direction of [2], we could also extrapolate to introduce {verify, update, create, delete} v2, which support both symrefs and regular refs. But require explicit types from the user:

    update-v2 SP <ref> NUL (oid <new-oid> | ref <new-target>) NUL
[(oid <old-oid> | ref <old-target>)] NUL
	create-v2 SP <ref> NUL (oid <new-oid> | ref <new-target>) NUL
	delete-v2 SP <ref> NUL [(oid <old-oid> | ref <old-target>)] NUL
	verify-v2 SP <ref> NUL [(oid <old-oid> | ref <old-target>)] NUL

This is similar to the v3 patches I've currently sent out, in that it would also allow cross operations between regular refs and symrefs.