Re: bw/transport-protocol-policy

From: Jeff King <peff@peff.net>
Date: Dec 1, 2016, 19:20 UTC
Message-ID: <20161201192055.44mtvtacyhpbqbqu@sigill.intra.peff.net>
In-Reply-To: <20161201181415.GA54082@google.com>

On Thu, Dec 01, 2016 at 10:14:15AM -0800, Brandon Williams wrote:

Show 38 quoted lines

> >   1. The new policy config lets you say "only allow this protocol when
> >      the user specifies it". But when http.c calls is_transport_allowed(),
> >      the latter has no idea that we are asking it about potential
> >      redirects (which obviously do _not_ come from the user), and would
> >      erroneously allow them.
> > 
> >      I think this needs fixed before the topic is merged. It's not a
> >      regression, as it only comes into play if you use the new policy
> >      config. But it is a minor security hole in the new feature.
> 
> I agree and it should be an easy fix.  We can just add a parameter like
> so:
> 
> diff --git a/transport.c b/transport.c
> index 2c0ec76..d38d50f 100644
> --- a/transport.c
> +++ b/transport.c
> @@ -723,7 +723,7 @@ static enum protocol_allow_config get_protocol_config(const char *type)
>  	return PROTOCOL_ALLOW_USER_ONLY;
>  }
>  
> -int is_transport_allowed(const char *type)
> +int is_transport_allowed(const char *type, int redirect)
>  {
>  	const struct string_list *whitelist = protocol_whitelist();
>  	if (whitelist)
> @@ -735,7 +735,7 @@ int is_transport_allowed(const char *type)
>  	case PROTOCOL_ALLOW_NEVER:
>  		return 0;
>  	case PROTOCOL_ALLOW_USER_ONLY:
> -		return git_env_bool("GIT_PROTOCOL_FROM_USER", 1);
> +		return git_env_bool("GIT_PROTOCOL_FROM_USER", !redirect);
>  	}
>  
>  	die("BUG: invalid protocol_allow_config type");
> 
> That way the libcurl code can say it is asking if it is ok to redirect
> to that protocol.

I wouldn't expect anyone to ever set GIT_PROTOCOL_FROM_USER=1, but it does behave in a funny way here, overriding the "redirect" flag. I think we'd want something more like:

  if (redirect < 0)
	redirect = git_env_bool("GIT_PROTOCOL_FROM_USER", 1);

and then pass in "-1" from transport_check_allowed().

I think that's sufficient to fix the topic as-is. However, the http redirect series adds an extra complication, because with http-alternates we resolve some of the redirects ourselves. So in those cases we'd want to restrict CURLOPT_PROTOCOLS as if they were redirects. We may need to set up two CURLOPT values: ones from the user and ones from redirects, and then feed them to CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS as appropriate depending on the request context.

> We should switch to warning all the time since this series adds in
> default whitelisted/blacklisted protocols anyways.

Yeah, good point. As a bonus it makes the code simpler.

-Peff