transport: make protocol.file.allow be "user" by default

An earlier patch discussed and fixed a scenario where Git could be used
as a vector to exfiltrate sensitive data through a Docker container when
a potential victim clones a suspicious repository with local submodules
that contain symlinks.

That security hole has since been plugged, but a similar one still
exists.  Instead of convincing a would-be victim to clone an embedded
submodule via the "file" protocol, an attacker could convince an
individual to clone a repository that has a submodule pointing to a
valid path on the victim's filesystem.

For example, if an individual (with username "foo") has their home
directory ("/home/foo") stored as a Git repository, then an attacker
could exfiltrate data by convincing a victim to clone a malicious
repository containing a submodule pointing at "/home/foo/.git" with
`--recurse-submodules`. Doing so would expose any sensitive contents in
stored in "/home/foo" tracked in Git.

For systems (such as Docker) that consider everything outside of the
immediate top-level working directory containing a Dockerfile as
inaccessible to the container (with the exception of volume mounts, and
so on), this is a violation of trust by exposing unexpected contents in
the working copy.

To mitigate the likelihood of this kind of attack, adjust the "file://"
protocol's default policy to be "user" to prevent commands that execute
without user input (including recursive submodule initialization) from
taking place by default.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/config/protocol.txt

@@ -1,10 +1,10 @@
 protocol.allow::
 	If set, provide a user defined default policy for all protocols which
 	don't explicitly have a policy (`protocol.<name>.allow`).  By default,
-	if unset, known-safe protocols (http, https, git, ssh, file) have a
+	if unset, known-safe protocols (http, https, git, ssh) have a
 	default policy of `always`, known-dangerous protocols (ext) have a
-	default policy of `never`, and all other protocols have a default
-	policy of `user`.  Supported policies:
+	default policy of `never`, and all other protocols (including file)
+	have a default policy of `user`.  Supported policies:
 +
 --
 

transport.c

@@ -964,8 +964,7 @@ static enum protocol_allow_config get_protocol_config(const char *type)
 	if (!strcmp(type, "http") ||
 	    !strcmp(type, "https") ||
 	    !strcmp(type, "git") ||
-	    !strcmp(type, "ssh") ||
-	    !strcmp(type, "file"))
+	    !strcmp(type, "ssh"))
 		return PROTOCOL_ALLOW_ALWAYS;
 
 	/* known scary; err on the side of caution */