http: limit redirection to protocol-whitelist

Previously, libcurl would follow redirection to any protocol
it was compiled for support with. This is desirable to allow
redirection from HTTP to HTTPS. However, it would even
successfully allow redirection from HTTP to SFTP, a protocol
that git does not otherwise support at all. Furthermore
git's new protocol-whitelisting could be bypassed by
following a redirect within the remote helper, as it was
only enforced at transport selection time.

This patch limits redirects within libcurl to HTTP, HTTPS,
FTP and FTPS. If there is a protocol-whitelist present, this
list is limited to those also allowed by the whitelist. As
redirection happens from within libcurl, it is impossible
for an HTTP redirect to a protocol implemented within
another remote helper.

When the curl version git was compiled with is too old to
support restrictions on protocol redirection, we warn the
user if GIT_ALLOW_PROTOCOL restrictions were requested. This
is a little inaccurate, as even without that variable in the
environment, we would still restrict SFTP, etc, and we do
not warn in that case. But anything else means we would
literally warn every time git accesses an http remote.

This commit includes a test, but it is not as robust as we
would hope. It redirects an http request to ftp, and checks
that curl complained about the protocol, which means that we
are relying on curl's specific error message to know what
happened. Ideally we would redirect to a working ftp server
and confirm that we can clone without protocol restrictions,
and not with them. But we do not have a portable way of
providing an ftp server, nor any other protocol that curl
supports (https is the closest, but we would have to deal
with certificates).

[jk: added test and version warning]

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 27 insertions, 5 deletions

diff --git a/Documentation/git.txt b/Documentation/git.txt
index b6a12b32ee..41a09cac7a 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -1071,11 +1071,6 @@ GIT_ICASE_PATHSPECS::
 
 	  - any external helpers are named by their protocol (e.g., use
 	    `hg` to allow the `git-remote-hg` helper)
-+
-Note that this controls only git's internal protocol selection.
-If libcurl is used (e.g., by the `http` transport), it may
-redirect to other protocols. There is not currently any way to
-restrict this.
 
 
 Discussion[[Discussion]]
diff --git a/http.c b/http.c
index 6798620065..5a57bccea9 100644
--- a/http.c
+++ b/http.c
@@ -8,6 +8,7 @@
 #include "credential.h"
 #include "version.h"
 #include "pkt-line.h"
+#include "transport.h"
 
 int active_requests;
 int http_is_verbose;
@@ -303,6 +304,7 @@ static void set_curl_keepalive(CURL *c)
 static CURL *get_curl_handle(void)
 {
 	CURL *result = curl_easy_init();
+	long allowed_protocols = 0;
 
 	if (!result)
 		die("curl_easy_init failed");
@@ -355,6 +357,21 @@ static CURL *get_curl_handle(void)
 #elif LIBCURL_VERSION_NUM >= 0x071101
 	curl_easy_setopt(result, CURLOPT_POST301, 1);
 #endif
+#if LIBCURL_VERSION_NUM >= 0x071304
+	if (is_transport_allowed("http"))
+		allowed_protocols |= CURLPROTO_HTTP;
+	if (is_transport_allowed("https"))
+		allowed_protocols |= CURLPROTO_HTTPS;
+	if (is_transport_allowed("ftp"))
+		allowed_protocols |= CURLPROTO_FTP;
+	if (is_transport_allowed("ftps"))
+		allowed_protocols |= CURLPROTO_FTPS;
+	curl_easy_setopt(result, CURLOPT_REDIR_PROTOCOLS, allowed_protocols);
+#else
+	if (transport_restrict_protocols())
+		warning("protocol restrictions not applied to curl redirects because\n"
+			"your curl version is too old (>= 7.19.4)");
+#endif
 
 	if (getenv("GIT_CURL_VERBOSE"))
 		curl_easy_setopt(result, CURLOPT_VERBOSE, 1);
diff --git a/t/lib-httpd/apache.conf b/t/lib-httpd/apache.conf
index 0b81a0047b..68ef8adb8e 100644
--- a/t/lib-httpd/apache.conf
+++ b/t/lib-httpd/apache.conf
@@ -119,6 +119,7 @@ RewriteRule ^/smart-redir-perm/(.*)$ /smart/$1 [R=301]
 RewriteRule ^/smart-redir-temp/(.*)$ /smart/$1 [R=302]
 RewriteRule ^/smart-redir-auth/(.*)$ /auth/smart/$1 [R=301]
 RewriteRule ^/smart-redir-limited/(.*)/info/refs$ /smart/$1/info/refs [R=301]
+RewriteRule ^/ftp-redir/(.*)$ ftp://localhost:1000/$1 [R=302]
 
 <IfDefine SSL>
 LoadModule ssl_module modules/mod_ssl.so
diff --git a/t/t5812-proto-disable-http.sh b/t/t5812-proto-disable-http.sh
index dd5001cbac..6a4f81662d 100755
--- a/t/t5812-proto-disable-http.sh
+++ b/t/t5812-proto-disable-http.sh
@@ -16,5 +16,14 @@ test_expect_success 'create git-accessible repo' '
 
 test_proto "smart http" http "$HTTPD_URL/smart/repo.git"
 
+test_expect_success 'curl redirects respect whitelist' '
+	test_must_fail env GIT_ALLOW_PROTOCOL=http:https \
+		git clone "$HTTPD_URL/ftp-redir/repo.git" 2>stderr &&
+	{
+		test_i18ngrep "ftp.*disabled" stderr ||
+		test_i18ngrep "your curl version is too old"
+	}
+'
+
 stop_httpd
 test_done