mingw: fix quoting of arguments

We need to be careful to follow proper quoting rules. For example, if an
argument contains spaces, we have to quote them. Double-quotes need to
be escaped. Backslashes need to be escaped, but only if they are
followed by a double-quote character.

We need to be _extra_ careful to consider the case where an argument
ends in a backslash _and_ needs to be quoted: in this case, we append a
double-quote character, i.e. the backslash now has to be escaped!

The current code, however, fails to recognize that, and therefore can
turn an argument that ends in a single backslash into a quoted argument
that now ends in an escaped double-quote character. This allows
subsequent command-line parameters to be split and part of them being
mistaken for command-line options, e.g. through a maliciously-crafted
submodule URL during a recursive clone.

Technically, we would not need to quote _all_ arguments which end in a
backslash _unless_ the argument needs to be quoted anyway. For example,
`test\` would not need to be quoted, while `test \` would need to be.

To keep the code simple, however, and therefore easier to reason about
and ensure its correctness, we now _always_ quote an argument that ends
in a backslash.

This addresses CVE-2019-1350.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

2 files changed, 20 insertions, 3 deletions

diff --git a/compat/mingw.c b/compat/mingw.c
index 8b6fa0db44..459ee20df6 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -872,7 +872,7 @@ static const char *quote_arg(const char *arg)
 				p++;
 				len++;
 			}
-			if (*p == '"')
+			if (*p == '"' || !*p)
 				n += count*2 + 1;
 			continue;
 		}
@@ -894,16 +894,19 @@ static const char *quote_arg(const char *arg)
 				count++;
 				*d++ = *arg++;
 			}
-			if (*arg == '"') {
+			if (*arg == '"' || !*arg) {
 				while (count-- > 0)
 					*d++ = '\\';
+				/* don't escape the surrounding end quote */
+				if (!*arg)
+					break;
 				*d++ = '\\';
 			}
 		}
 		*d++ = *arg++;
 	}
 	*d++ = '"';
-	*d++ = 0;
+	*d++ = '\0';
 	return q;
 }
 
diff --git a/t/t7416-submodule-dash-url.sh b/t/t7416-submodule-dash-url.sh
index 459193c976..2966e93071 100755
--- a/t/t7416-submodule-dash-url.sh
+++ b/t/t7416-submodule-dash-url.sh
@@ -31,4 +31,18 @@ test_expect_success 'clone rejects unprotected dash' '
 	test_i18ngrep ignoring err
 '
 
+test_expect_success 'trailing backslash is handled correctly' '
+	git init testmodule &&
+	test_commit -C testmodule c &&
+	git submodule add ./testmodule &&
+	: ensure that the name ends in a double backslash &&
+	sed -e "s|\\(submodule \"testmodule\\)\"|\\1\\\\\\\\\"|" \
+		-e "s|url = .*|url = \" --should-not-be-an-option\"|" \
+		<.gitmodules >.new &&
+	mv .new .gitmodules &&
+	git commit -am "Add testmodule" &&
+	test_must_fail git clone --verbose --recurse-submodules . dolly 2>err &&
+	test_i18ngrep ! "unknown option" err
+'
+
 test_done