Disallow NTFS Alternate Data Stream attacks, even on Linux/macOS

A little-known feature of NTFS is that it offers to store metadata in
so-called "Alternate Data Streams" (inspired by Apple's "resource
forks") that are copied together with the file they are associated with.
These Alternate Data Streams can be accessed via `<file name>:<stream
name>:<stream type>`.

Directories, too, have Alternate Data Streams, and they even have a
default stream type `$INDEX_ALLOCATION`. Which means that `abc/` and
`abc::$INDEX_ALLOCATION/` are actually equivalent.

This is of course another attack vector on the Git directory that we
definitely want to prevent.

On Windows, we already do this incidentally, by disallowing colons in
file/directory names.

While it looks as if files'/directories' Alternate Data Streams are not
accessible in the Windows Subsystem for Linux, and neither via
CIFS/SMB-mounted network shares in Linux, it _is_ possible to access
them on SMB-mounted network shares on macOS.

Therefore, let's go the extra mile and prevent this particular attack
_everywhere_. To keep things simple, let's just disallow *any* Alternate
Data Stream of `.git`.

This is libgit2's variant of CVE-2019-1352.

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

6 files changed, 17 insertions, 2 deletions

diff --git a/src/path.c b/src/path.c
index 90473fc19..32fcfa1fa 100644
--- a/src/path.c
+++ b/src/path.c
@@ -1609,8 +1609,12 @@ GIT_INLINE(bool) verify_dotgit_ntfs(git_repository *repo, const char *path, size
 	if (!start)
 		return true;
 
-	/* Reject paths like ".git\" */
-	if (path[start] == '\\')
+	/*
+	 * Reject paths that start with Windows-style directory separators
+	 * (".git\") or NTFS alternate streams (".git:") and could be used
+	 * to write to the ".git" directory on Windows platforms.
+	 */
+	if (path[start] == '\\' || path[start] == ':')
 		return false;
 
 	/* Reject paths like '.git ' or '.git.' */
diff --git a/tests/checkout/nasty.c b/tests/checkout/nasty.c
index 99292206f..288341a53 100644
--- a/tests/checkout/nasty.c
+++ b/tests/checkout/nasty.c
@@ -273,6 +273,16 @@ void test_checkout_nasty__dot_git_colon_stuff(void)
 #endif
 }
 
+/* A tree that contains an entry ".git::$INDEX_ALLOCATION" because NTFS
+ * will interpret that as a synonym to ".git", even when mounted via SMB
+ * on macOS.
+ */
+void test_checkout_nasty__dotgit_alternate_data_stream(void)
+{
+	test_checkout_fails("refs/heads/dotgit_alternate_data_stream", ".git/dummy-file");
+	test_checkout_fails("refs/heads/dotgit_alternate_data_stream", ".git::$INDEX_ALLOCATION/dummy-file");
+}
+
 /* Trees that contains entries with a tree ".git" that contain
  * byte sequences:
  * { 0xe2, 0x80, 0x8c }
diff --git a/tests/resources/nasty/.gitted/objects/33/8190107c7ee7d8f5aa30061fc19b7d5ddcda86 b/tests/resources/nasty/.gitted/objects/33/8190107c7ee7d8f5aa30061fc19b7d5ddcda86
new file mode 100644
index 000000000..e539ccfec
--- /dev/null
+++ b/tests/resources/nasty/.gitted/objects/33/8190107c7ee7d8f5aa30061fc19b7d5ddcda86
diff --git a/tests/resources/nasty/.gitted/objects/97/c14994866466aeb73e769a6f34e07c7f4b53f7 b/tests/resources/nasty/.gitted/objects/97/c14994866466aeb73e769a6f34e07c7f4b53f7
new file mode 100644
index 000000000..9f7679917
--- /dev/null
+++ b/tests/resources/nasty/.gitted/objects/97/c14994866466aeb73e769a6f34e07c7f4b53f7
diff --git a/tests/resources/nasty/.gitted/objects/b8/edf3ad62dbcbc983857a5bfee7b0181ee1a513 b/tests/resources/nasty/.gitted/objects/b8/edf3ad62dbcbc983857a5bfee7b0181ee1a513
new file mode 100644
index 000000000..bf446263c
--- /dev/null
+++ b/tests/resources/nasty/.gitted/objects/b8/edf3ad62dbcbc983857a5bfee7b0181ee1a513
diff --git a/tests/resources/nasty/.gitted/refs/heads/dotgit_alternate_data_stream b/tests/resources/nasty/.gitted/refs/heads/dotgit_alternate_data_stream
new file mode 100644
index 000000000..ecdd340cd
--- /dev/null
+++ b/tests/resources/nasty/.gitted/refs/heads/dotgit_alternate_data_stream
@@ -0,0 +1 @@
+b8edf3ad62dbcbc983857a5bfee7b0181ee1a513