transport: add a protocol-whitelist environment variable

If we are cloning an untrusted remote repository into a
sandbox, we may also want to fetch remote submodules in
order to get the complete view as intended by the other
side. However, that opens us up to attacks where a malicious
user gets us to clone something they would not otherwise
have access to (this is not necessarily a problem by itself,
but we may then act on the cloned contents in a way that
exposes them to the attacker).

Ideally such a setup would sandbox git entirely away from
high-value items, but this is not always practical or easy
to set up (e.g., OS network controls may block multiple
protocols, and we would want to enable some but not others).

We can help this case by providing a way to restrict
particular protocols. We use a whitelist in the environment.
This is more annoying to set up than a blacklist, but
defaults to safety if the set of protocols git supports
grows). If no whitelist is specified, we continue to default
to allowing all protocols (this is an "unsafe" default, but
since the minority of users will want this sandboxing
effect, it is the only sensible one).

A note on the tests: ideally these would all be in a single
test file, but the git-daemon and httpd test infrastructure
is an all-or-nothing proposition rather than a test-by-test
prerequisite. By putting them all together, we would be
unable to test the file-local code on machines without
apache.

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

1 files changed, 32 insertions, 0 deletions

diff --git a/Documentation/git.txt b/Documentation/git.txt
index a62ed6f..b6a12b3 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -1045,6 +1045,38 @@ GIT_ICASE_PATHSPECS::
 	an operation has touched every ref (e.g., because you are
 	cloning a repository to make a backup).
 
+`GIT_ALLOW_PROTOCOL`::
+	If set, 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.  The protocol names
+	currently used by git are:
+
+	  - `file`: any local file-based path (including `file://` URLs,
+	    or local paths)
+
+	  - `git`: the anonymous git protocol over a direct TCP
+	    connection (or proxy, if configured)
+
+	  - `ssh`: git over ssh (including `host:path` syntax,
+	    `git+ssh://`, etc).
+
+	  - `rsync`: git over rsync
+
+	  - `http`: git over http, both "smart http" and "dumb http".
+	    Note that this does _not_ include `https`; if you want both,
+	    you should specify both as `http:https`.
+
+	  - 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]]
 ------------------------