shell: drop git-cvsserver support by default

The git-cvsserver script is old and largely unmaintained
these days. But git-shell allows untrusted users to run it
out of the box, significantly increasing its attack surface.

Let's drop it from git-shell's list of internal handlers so
that it cannot be run by default.  This is not backwards
compatible. But given the age and development activity on
CVS-related parts of Git, this is likely to impact very few
users, while helping many more (i.e., anybody who runs
git-shell and had no intention of supporting CVS).

There's no configuration mechanism in git-shell for us to
add a boolean and flip it to "off". But there is a mechanism
for adding custom commands, and adding CVS support here is
fairly trivial. Let's document it to give guidance to
anybody who really is still running cvsserver.

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

Documentation/git-shell.txt

@@ -79,6 +79,22 @@ EOF
 $ chmod +x $HOME/git-shell-commands/no-interactive-login
 ----------------
 
+To enable git-cvsserver access (which should generally have the
+`no-interactive-login` example above as a prerequisite, as creating
+the git-shell-commands directory allows interactive logins):
+
+----------------
+$ cat >$HOME/git-shell-commands/cvs <<\EOF
+if ! test $# = 1 && test "$1" = "server"
+then
+	echo >&2 "git-cvsserver only handles \"server\""
+	exit 1
+fi
+exec git cvsserver server
+EOF
+$ chmod +x $HOME/git-shell-commands/cvs
+----------------
+
 SEE ALSO
 --------
 ssh(1),

shell.c

@@ -25,19 +25,6 @@ static int do_generic_cmd(const char *me, char *arg)
 	return execv_git_cmd(my_argv);
 }
 
-static int do_cvs_cmd(const char *me, char *arg)
-{
-	const char *cvsserver_argv[3] = {
-		"cvsserver", "server", NULL
-	};
-
-	if (!arg || strcmp(arg, "server"))
-		die("git-cvsserver only handles server: %s", arg);
-
-	setup_path();
-	return execv_git_cmd(cvsserver_argv);
-}
-
 static int is_valid_cmd_name(const char *cmd)
 {
 	/* Test command contains no . or / characters */
@@ -134,7 +121,6 @@ static struct commands {
 	{ "git-receive-pack", do_generic_cmd },
 	{ "git-upload-pack", do_generic_cmd },
 	{ "git-upload-archive", do_generic_cmd },
-	{ "cvs", do_cvs_cmd },
 	{ NULL },
 };
 

t/t9400-git-cvsserver-server.sh

@@ -588,4 +588,52 @@ test_expect_success 'cvs annotate' '
     test_cmp ../expect ../actual
 '
 
+#------------
+# running via git-shell
+#------------
+
+cd "$WORKDIR"
+
+test_expect_success 'create remote-cvs helper' '
+	write_script remote-cvs <<-\EOF
+	exec git shell -c "cvs server"
+	EOF
+'
+
+test_expect_success 'cvs server does not run with vanilla git-shell' '
+	(
+		cd cvswork &&
+		CVS_SERVER=$WORKDIR/remote-cvs &&
+		export CVS_SERVER &&
+		test_must_fail cvs log merge
+	)
+'
+
+test_expect_success 'configure git shell to run cvs server' '
+	mkdir "$HOME"/git-shell-commands &&
+
+	write_script "$HOME"/git-shell-commands/cvs <<-\EOF &&
+	if ! test $# = 1 && test "$1" = "server"
+	then
+		echo >&2 "git-cvsserver only handles \"server\""
+		exit 1
+	fi
+	exec git cvsserver server
+	EOF
+
+	# Should not be used, but part of the recommended setup
+	write_script "$HOME"/git-shell-commands/no-interactive-login <<-\EOF
+	echo Interactive login forbidden
+	EOF
+'
+
+test_expect_success 'cvs server can run with recommended config' '
+	(
+		cd cvswork &&
+		CVS_SERVER=$WORKDIR/remote-cvs &&
+		export CVS_SERVER &&
+		cvs log merge
+	)
+'
+
 test_done