From a94971b023f4583ea4b2c9c9f7a71d53f9781d58 Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Fri, 6 Sep 2019 00:09:10 +0200
Subject: [PATCH] mingw: handle `subst`-ed "DOS drives"
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Over a decade ago, in 25fe217b86c (Windows: Treat Windows style path
names., 2008-03-05), Git was taught to handle absolute Windows paths,
i.e. paths that start with a drive letter and a colon.

Unbeknownst to us, while drive letters of physical drives are limited to
letters of the English alphabet, there is a way to assign virtual drive
letters to arbitrary directories, via the `subst` command, which is
_not_ limited to English letters.

It is therefore possible to have absolute Windows paths of the form
`1:\what\the\hex.txt`. Even "better": pretty much arbitrary Unicode
letters can also be used, e.g. `ä:\tschibät.sch`.

While it can be sensibly argued that users who set up such funny drive
letters really seek adverse consequences, the Windows Operating System
is known to be a platform where many users are at the mercy of
administrators who have their very own idea of what constitutes a
reasonable setup.

Therefore, let's just make sure that such funny paths are still
considered absolute paths by Git, on Windows.

In addition to Unicode characters, pretty much any character is a valid
drive letter, as far as `subst` is concerned, even `:` and `"` or even a
space character. While it is probably the opposite of smart to use them,
let's safeguard `is_dos_drive_prefix()` against all of them.

Note: `[::1]:repo` is a valid URL, but not a valid path on Windows.
As `[` is now considered a valid drive letter, we need to be very
careful to avoid misinterpreting such a string as valid local path in
`url_is_local_not_ssh()`. To do that, we use the just-introduced
function `is_valid_path()` (which will label the string as invalid file
name because of the colon characters).

This fixes CVE-2019-1351.

Reported-by: Nicolas Joly <Nicolas.Joly@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/win32/path-utils.c | 24 ++++++++++++++++++++++++
 compat/win32/path-utils.h |  4 ++--
 connect.c                 |  2 +-
 t/t0060-path-utils.sh     |  9 +++++++++
 4 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/compat/win32/path-utils.c b/compat/win32/path-utils.c
index d9d3641de8..70352af283 100644
--- a/compat/win32/path-utils.c
+++ b/compat/win32/path-utils.c
@@ -1,5 +1,29 @@
 #include "../../git-compat-util.h"
 
+int mingw_has_dos_drive_prefix(const char *path)
+{
+	int i;
+
+	/*
+	* Does it start with an ASCII letter (i.e. highest bit not set),
+	* followed by a colon?
+	*/
+	if (!(0x80 & (unsigned char)*path))
+		return *path && path[1] == ':' ? 2 : 0;
+
+	/*
+	 * While drive letters must be letters of the English alphabet, it is
+	 * possible to assign virtually _any_ Unicode character via `subst` as
+	 * a drive letter to "virtual drives". Even `1`, or `ä`. Or fun stuff
+	 * like this:
+	 *
+	 *      subst ֍: %USERPROFILE%\Desktop
+	 */
+	for (i = 1; i < 4 && (0x80 & (unsigned char)path[i]); i++)
+		; /* skip first UTF-8 character */
+	return path[i] == ':' ? i + 1 : 0;
+}
+
 int win32_skip_dos_drive_prefix(char **path)
 {
 	int ret = has_dos_drive_prefix(*path);
diff --git a/compat/win32/path-utils.h b/compat/win32/path-utils.h
index 0f70d43920..22b0c63349 100644
--- a/compat/win32/path-utils.h
+++ b/compat/win32/path-utils.h
@@ -1,5 +1,5 @@
-#define has_dos_drive_prefix(path) \
-	(isalpha(*(path)) && (path)[1] == ':' ? 2 : 0)
+int mingw_has_dos_drive_prefix(const char *path);
+#define has_dos_drive_prefix mingw_has_dos_drive_prefix
 int win32_skip_dos_drive_prefix(char **path);
 #define skip_dos_drive_prefix win32_skip_dos_drive_prefix
 static inline int win32_is_dir_sep(int c)
diff --git a/connect.c b/connect.c
index 2778481264..4050a797bf 100644
--- a/connect.c
+++ b/connect.c
@@ -511,7 +511,7 @@ int url_is_local_not_ssh(const char *url)
 	const char *colon = strchr(url, ':');
 	const char *slash = strchr(url, '/');
 	return !colon || (slash && slash < colon) ||
-		has_dos_drive_prefix(url);
+		(has_dos_drive_prefix(url) && is_valid_path(url));
 }
 
 static const char *prot_name(enum protocol protocol)
diff --git a/t/t0060-path-utils.sh b/t/t0060-path-utils.sh
index c7b53e494b..e9b61efdff 100755
--- a/t/t0060-path-utils.sh
+++ b/t/t0060-path-utils.sh
@@ -165,6 +165,15 @@ test_expect_success 'absolute path rejects the empty string' '
 	test_must_fail test-tool path-utils absolute_path ""
 '
 
+test_expect_success MINGW '<drive-letter>:\\abc is an absolute path' '
+	for letter in : \" C Z 1 ä
+	do
+		path=$letter:\\abc &&
+		absolute="$(test-path-utils absolute_path "$path")" &&
+		test "$path" = "$absolute" || return 1
+	done
+'
+
 test_expect_success 'real path rejects the empty string' '
 	test_must_fail test-tool path-utils real_path ""
 '
-- 
2.23.0

