fsck: reject URL with empty host in .gitmodules

Git's URL parser interprets

	https:///example.com/repo.git

to have no host and a path of "example.com/repo.git".  Curl, on the
other hand, internally redirects it to https://example.com/repo.git.  As
a result, until "credential: parse URL without host as empty host, not
unset", tricking a user into fetching from such a URL would cause Git to
send credentials for another host to example.com.

Teach fsck to block and detect .gitmodules files using such a URL to
prevent sharing them with Git versions that are not yet protected.

A relative URL in a .gitmodules file could also be used to trigger this.
The relative URL resolver used for .gitmodules does not normalize
sequences of slashes and can follow ".." components out of the path part
and to the host part of a URL, meaning that such a relative URL can be
used to traverse from a https://foo.example.com/innocent superproject to
a https:///attacker.example.com/exploit submodule. Fortunately,
redundant extra slashes in .gitmodules are rare, so we can catch this by
detecting one after a leading sequence of "./" and "../" components.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>

fsck.c

@@ -1064,17 +1064,21 @@ static int check_submodule_url(const char *url)
 		/*
 		 * URLs which escape their root via "../" can overwrite
 		 * the host field and previous components, resolving to
-		 * URLs like https::example.com/submodule.git that were
+		 * URLs like https::example.com/submodule.git and
+		 * https:///example.com/submodule.git that were
 		 * susceptible to CVE-2020-11008.
 		 */
 		if (count_leading_dotdots(url, &next) > 0 &&
-		    *next == ':')
+		    (*next == ':' || *next == '/'))
 			return -1;
 	}
 
 	else if (url_to_curl_url(url, &curl_url)) {
 		struct credential c = CREDENTIAL_INIT;
-		int ret = credential_from_url_gently(&c, curl_url, 1);
+		int ret = 0;
+		if (credential_from_url_gently(&c, curl_url, 1) ||
+		    !*c.host)
+			ret = -1;
 		credential_clear(&c);
 		return ret;
 	}

t/t7416-submodule-dash-url.sh

@@ -124,6 +124,38 @@ test_expect_success 'fsck rejects relative URL resolving to empty scheme' '
 	grep gitmodulesUrl err
 '
 
+test_expect_success 'fsck rejects empty hostname' '
+	git checkout --orphan empty-host &&
+	cat >.gitmodules <<-\EOF &&
+	[submodule "foo"]
+		url = http:///one.example.com/foo.git
+	EOF
+	git add .gitmodules &&
+	test_tick &&
+	git commit -m "gitmodules with extra slashes" &&
+	test_when_finished "rm -rf dst" &&
+	git init --bare dst &&
+	git -C dst config transfer.fsckObjects true &&
+	test_must_fail git push dst HEAD 2>err &&
+	grep gitmodulesUrl err
+'
+
+test_expect_success 'fsck rejects relative url that produced empty hostname' '
+	git checkout --orphan messy-relative &&
+	cat >.gitmodules <<-\EOF &&
+	[submodule "foo"]
+		url = ../../..//one.example.com/foo.git
+	EOF
+	git add .gitmodules &&
+	test_tick &&
+	git commit -m "gitmodules abusing relative_path" &&
+	test_when_finished "rm -rf dst" &&
+	git init --bare dst &&
+	git -C dst config transfer.fsckObjects true &&
+	test_must_fail git push dst HEAD 2>err &&
+	grep gitmodulesUrl err
+'
+
 test_expect_success 'fsck permits embedded newline with unrecognized scheme' '
 	git checkout --orphan newscheme &&
 	cat >.gitmodules <<-\EOF &&