[PATCH 2/3] credential: teach `credential_from_url()` a non-strict mode

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

Prior to the fixes for CVE-2020-11008, we were _very_ lenient in what we
required from a URL in order to parse it into a `struct credential`.
That led to serious vulnerabilities.

There was one call site, though, that really needed that leniency: when
parsing config settings a la `credential.dev.azure.com.useHTTPPath`.

In preparation for fixing that regression, let's add a parameter called
`strict` to the `credential_from_url()` function and convert the
existing callers to enforce that strict mode.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 credential.c | 14 ++++++++------
 credential.h |  6 +++++-
 fsck.c       |  2 +-
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/credential.c b/credential.c
index 64a841eddca..c73260ac40f 100644
--- a/credential.c
+++ b/credential.c
@@ -344,7 +344,7 @@ static int check_url_component(const char *url, int quiet,
 }
 
 int credential_from_url_gently(struct credential *c, const char *url,
-			       int quiet)
+			       int strict, int quiet)
 {
 	const char *at, *colon, *cp, *slash, *host, *proto_end;
 
@@ -357,12 +357,12 @@ int credential_from_url_gently(struct credential *c, const char *url,
 	 *   (3) proto://<user>:<pass>@<host>/...
 	 */
 	proto_end = strstr(url, "://");
-	if (!proto_end || proto_end == url) {
+	if (strict && (!proto_end || proto_end == url)) {
 		if (!quiet)
 			warning(_("url has no scheme: %s"), url);
 		return -1;
 	}
-	cp = proto_end + 3;
+	cp = proto_end ? proto_end + 3 : url;
 	at = strchr(cp, '@');
 	colon = strchr(cp, ':');
 	slash = strchrnul(cp, '/');
@@ -382,8 +382,10 @@ int credential_from_url_gently(struct credential *c, const char *url,
 		host = at + 1;
 	}
 
-	c->protocol = xmemdupz(url, proto_end - url);
-	c->host = url_decode_mem(host, slash - host);
+	if (proto_end && proto_end - url > 0)
+		c->protocol = xmemdupz(url, proto_end - url);
+	if (slash - url > 0)
+		c->host = url_decode_mem(host, slash - host);
 	/* Trim leading and trailing slashes from path */
 	while (*slash == '/')
 		slash++;
@@ -407,6 +409,6 @@ int credential_from_url_gently(struct credential *c, const char *url,
 
 void credential_from_url(struct credential *c, const char *url)
 {
-	if (credential_from_url_gently(c, url, 0) < 0)
+	if (credential_from_url_gently(c, url, 1, 0) < 0)
 		die(_("credential url cannot be parsed: %s"), url);
 }
diff --git a/credential.h b/credential.h
index 5a86502d95c..823ec2caf35 100644
--- a/credential.h
+++ b/credential.h
@@ -41,9 +41,13 @@ void credential_write(const struct credential *, FILE *);
  * an error but leave the broken state in the credential object for further
  * examination.  The non-gentle form will issue a warning to stderr and return
  * an empty credential.
+ *
+ * In strict mode, an empty protocol or an empty host name are not allowed.
+ * The credential_from_url() function enforces strict mode.
  */
 void credential_from_url(struct credential *, const char *url);
-int credential_from_url_gently(struct credential *, const char *url, int quiet);
+int credential_from_url_gently(struct credential *, const char *url,
+			       int strict, int quiet);
 
 int credential_match(const struct credential *have,
 		     const struct credential *want);
diff --git a/fsck.c b/fsck.c
index 31b5be05f54..aa66dc1e742 100644
--- a/fsck.c
+++ b/fsck.c
@@ -1076,7 +1076,7 @@ static int check_submodule_url(const char *url)
 	else if (url_to_curl_url(url, &curl_url)) {
 		struct credential c = CREDENTIAL_INIT;
 		int ret = 0;
-		if (credential_from_url_gently(&c, curl_url, 1) ||
+		if (credential_from_url_gently(&c, curl_url, 1, 1) ||
 		    !*c.host)
 			ret = -1;
 		credential_clear(&c);
-- 
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