path: safeguard `.git` against NTFS Alternate Streams Accesses

Probably inspired by HFS' resource streams, NTFS supports "Alternate
Data Streams": by appending `:<stream-name>` to the file name,
information in addition to the file contents can be written and read,
information that is copied together with the file (unless copied to a
non-NTFS location).

These Alternate Data Streams are typically used for things like marking
an executable as having just been downloaded from the internet (and
hence not necessarily being trustworthy).

In addition to a stream name, a stream type can be appended, like so:
`:<stream-name>:<stream-type>`. Unless specified, the default stream
type is `$DATA` for files and `$INDEX_ALLOCATION` for directories. In
other words, `.git::$INDEX_ALLOCATION` is a valid way to reference the
`.git` directory!

In our work in Git v2.2.1 to protect Git on NTFS drives under
`core.protectNTFS`, we focused exclusively on NTFS short names, unaware
of the fact that NTFS Alternate Data Streams offer a similar attack
vector.

Let's fix this.

Seeing as it is better to be safe than sorry, we simply disallow paths
referring to *any* NTFS Alternate Data Stream of `.git`, not just
`::$INDEX_ALLOCATION`. This also simplifies the implementation.

This closes CVE-2019-1352.

Further reading about NTFS Alternate Data Streams:
https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fscc/c54dec26-1551-4d3a-a0ea-4fa40f848eb3

Reported-by: Nicolas Joly <Nicolas.Joly@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

path.c

@@ -1321,10 +1321,19 @@ static int only_spaces_and_periods(const char *path, size_t len, size_t skip)
  *   `.git` is the first item in a directory, therefore it will be associated
  *   with the short name `git~1` (unless short names are disabled).
  *
+ * - For yet other historical reasons, NTFS supports so-called "Alternate Data
+ *   Streams", i.e. metadata associated with a given file, referred to via
+ *   `<filename>:<stream-name>:<stream-type>`. There exists a default stream
+ *   type for directories, allowing `.git/` to be accessed via
+ *   `.git::$INDEX_ALLOCATION/`.
+ *
  * When this function returns 1, it indicates that the specified file/directory
  * name refers to a `.git` file or directory, or to any of these synonyms, and
  * Git should therefore not track it.
  *
+ * For performance reasons, _all_ Alternate Data Streams of `.git/` are
+ * forbidden, not just `::$INDEX_ALLOCATION`.
+ *
  * This function is intended to be used by `git fsck` even on platforms where
  * the backslash is a regular filename character, therefore it needs to handle
  * backlash characters in the provided `name` specially: they are interpreted
@@ -1335,7 +1344,8 @@ int is_ntfs_dotgit(const char *name)
 	size_t len;
 
 	for (len = 0; ; len++)
-		if (!name[len] || name[len] == '\\' || is_dir_sep(name[len])) {
+		if (!name[len] || name[len] == '\\' || is_dir_sep(name[len]) ||
+		    name[len] == ':') {
 			if (only_spaces_and_periods(name, len, 4) &&
 					!strncasecmp(name, ".git", 4))
 				return 1;

t/t1014-read-tree-confusing.sh

@@ -49,6 +49,7 @@ git~1
 .git.SPACE .git.{space}
 .\\\\.GIT\\\\foobar backslashes
 .git\\\\foobar backslashes2
+.git...:alternate-stream
 EOF
 
 test_expect_success 'utf-8 paths allowed with core.protectHFS off' '