Re: [PATCH] transport: add core.allowProtocol config option

On 11/02, Jeff King wrote:

On Wed, Nov 02, 2016 at 03:20:47PM -0700, Brandon Williams wrote:

quoted

Add configuration option 'core.allowProtocol' to allow users to create a
whitelist of allowed protocols for fetch/push/clone in their gitconfig.

For git-submodule.sh, fallback to default whitelist only if the user
hasn't explicitly set `GIT_ALLOW_PROTOCOL` or doesn't have a whitelist
in their gitconfig.

This says "what", but not "why". What's the use case?

I can see somebody wanting to pare down the whitelist further (e.g.,
because they are carrying ssh credentials that they don't want to use on
behalf of a malicious repo). But in general I'd expect this setting to
be a function of the environment you're operating in, and not the
on-disk config.

Or is the intent to broaden it for cases where you have a clone that
uses some non-standard protocol, and you want it to Just Work on
subsequent recursive fetches?

quoted

+core.allowProtocol::
+	Provide a colon-separated list of protocols which are allowed to be
+	used with fetch/push/clone. This is useful to restrict recursive
+	submodule initialization from an untrusted repository. Any protocol not
+	mentioned will be disallowed (i.e., this is a whitelist, not a
+	blacklist). If the variable is not set at all, all protocols are
+	enabled. If the `GIT_ALLOW_PROTOCOL` enviornment variable is set, it is
+	used as the protocol whitelist instead of this config option.

The "not set at all, all protocols are enabled" bit is not quite
correct, is it? It is true for a top-level fetch, but not for submodule
recursion (and especially since you are talking about submodule
recursion immediately before, it is rather confusing).

Yeah stefan mentioned this to me.  I simply copied the documentaion from
GIT_ALLOW_PROTOCOL, perhaps that should be updated as well?

quoted

--- a/git-submodule.sh
+++ b/git-submodule.sh

@@ -27,7 +27,8 @@ cd_to_toplevel
 #
 # If the user has already specified a set of allowed protocols,
 # we assume they know what they're doing and use that instead.
-: ${GIT_ALLOW_PROTOCOL=file:git:http:https:ssh}
+config_whitelist=$(git config core.allowProtocol)
+: ${GIT_ALLOW_PROTOCOL=${config_whitelist:-file:git:http:https:ssh}}

The original uses "=" without a ":" so that an empty variable takes
precedence over the stock list (i.e., allowing nothing). Would you want
the same behavior for the config variable? I.e.:

  # this should probably allow nothing, right?
  git config core.allowProtocol ""

I think you'd have to check the return code of "git config" to
distinguish those cases.

Oh, I didn't think of that case.  That can be done easy enough, just
makes the code a bit more verbose.

quoted

diff --git a/transport.c b/transport.c
index d57e8de..b1098cd 100644
--- a/transport.c
+++ b/transport.c

@@ -652,7 +652,7 @@ static const struct string_list *protocol_whitelist(void)
 
 	if (enabled < 0) {
 		const char *v = getenv("GIT_ALLOW_PROTOCOL");
-		if (v) {
+		if (v || !git_config_get_value("core.allowProtocol", &v)) {
 			string_list_split(&allowed, v, ':', -1);
 			string_list_sort(&allowed);
 			enabled = 1;

I thought at first we'd have to deal with leaking "v", but "get_value"
is the "raw" version that gives you the uninterpreted value. I think
that means it may give you NULL, though if we see an implicit bool like:

  [core]
  allowProtocol

That's nonsense, of course, but we would still segfault. I
think the easiest way to test is:

  git -c core.allowProtocol fetch

which seems to segfault for me with this patch.

what is the desired behavior when a user provides a config in a way that
isn't intended?

-- 
Brandon Williams