mingw: refuse to access paths with trailing spaces or periods

When creating a directory on Windows whose path ends in a space or a
period (or chains thereof), the Win32 API "helpfully" trims those. For
example, `mkdir("abc ");` will return success, but actually create a
directory called `abc` instead.

This stems back to the DOS days, when all file names had exactly 8
characters plus exactly 3 characters for the file extension, and the
only way to have shorter names was by padding with spaces.

Sadly, this "helpful" behavior is a bit inconsistent: after a successful
`mkdir("abc ");`, a `mkdir("abc /def")` will actually _fail_ (because
the directory `abc ` does not actually exist).

Even if it would work, we now have a serious problem because a Git
repository could contain directories `abc` and `abc `, and on Windows,
they would be "merged" unintentionally.

As these paths are illegal on Windows, anyway, let's disallow any
accesses to such paths on that Operating System.

For practical reasons, this behavior is still guarded by the
config setting `core.protectNTFS`: it is possible (and at least two
regression tests make use of it) to create commits without involving the
worktree. In such a scenario, it is of course possible -- even on
Windows -- to create such file names.

Among other consequences, this patch disallows submodules' paths to end
in spaces on Windows (which would formerly have confused Git enough to
try to write into incorrect paths, anyway).

While this patch does not fix a vulnerability on its own, it prevents an
attack vector that was exploited in demonstrations of a number of
recently-fixed security bugs.

The regression test added to `t/t7417-submodule-path-url.sh` reflects
that attack vector.

Note that we have to adjust the test case "prevent git~1 squatting on
Windows" in `t/t7415-submodule-names.sh` because of a very subtle issue.
It tries to clone two submodules whose names differ only in a trailing
period character, and as a consequence their git directories differ in
the same way. Previously, when Git tried to clone the second submodule,
it thought that the git directory already existed (because on Windows,
when you create a directory with the name `b.` it actually creates `b`),
but with this patch, the first submodule's clone will fail because of
the illegal name of the git directory. Therefore, when cloning the
second submodule, Git will take a different code path: a fresh clone
(without an existing git directory). Both code paths fail to clone the
second submodule, both because the the corresponding worktree directory
exists and is not empty, but the error messages are worded differently.

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

2 files changed, 67 insertions, 1 deletions

diff --git a/compat/mingw.c b/compat/mingw.c
index 8b6fa0db44..17b4da16e8 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -333,6 +333,12 @@ int mingw_mkdir(const char *path, int mode)
 {
 	int ret;
 	wchar_t wpath[MAX_PATH];
+
+	if (!is_valid_win32_path(path)) {
+		errno = EINVAL;
+		return -1;
+	}
+
 	if (xutftowcs_path(wpath, path) < 0)
 		return -1;
 	ret = _wmkdir(wpath);
@@ -345,13 +351,18 @@ int mingw_open (const char *filename, int oflags, ...)
 {
 	va_list args;
 	unsigned mode;
-	int fd;
+	int fd, create = (oflags & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL);
 	wchar_t wfilename[MAX_PATH];
 
 	va_start(args, oflags);
 	mode = va_arg(args, int);
 	va_end(args);
 
+	if (!is_valid_win32_path(filename)) {
+		errno = create ? EINVAL : ENOENT;
+		return -1;
+	}
+
 	if (filename && !strcmp(filename, "/dev/null"))
 		filename = "nul";
 
@@ -413,6 +424,11 @@ FILE *mingw_fopen (const char *filename, const char *otype)
 	int hide = needs_hiding(filename);
 	FILE *file;
 	wchar_t wfilename[MAX_PATH], wotype[4];
+	if (!is_valid_win32_path(filename)) {
+		int create = otype && strchr(otype, 'w');
+		errno = create ? EINVAL : ENOENT;
+		return NULL;
+	}
 	if (filename && !strcmp(filename, "/dev/null"))
 		filename = "nul";
 	if (xutftowcs_path(wfilename, filename) < 0 ||
@@ -435,6 +451,11 @@ FILE *mingw_freopen (const char *filename, const char *otype, FILE *stream)
 	int hide = needs_hiding(filename);
 	FILE *file;
 	wchar_t wfilename[MAX_PATH], wotype[4];
+	if (!is_valid_win32_path(filename)) {
+		int create = otype && strchr(otype, 'w');
+		errno = create ? EINVAL : ENOENT;
+		return NULL;
+	}
 	if (filename && !strcmp(filename, "/dev/null"))
 		filename = "nul";
 	if (xutftowcs_path(wfilename, filename) < 0 ||
@@ -2106,6 +2127,40 @@ static void setup_windows_environment(void)
 		setenv("TERM", "cygwin", 1);
 }
 
+int is_valid_win32_path(const char *path)
+{
+	int preceding_space_or_period = 0, i = 0, periods = 0;
+
+	if (!protect_ntfs)
+		return 1;
+
+	for (;;) {
+		char c = *(path++);
+		switch (c) {
+		case '\0':
+		case '/': case '\\':
+			/* cannot end in ` ` or `.`, except for `.` and `..` */
+			if (preceding_space_or_period &&
+			    (i != periods || periods > 2))
+				return 0;
+			if (!c)
+				return 1;
+
+			i = periods = preceding_space_or_period = 0;
+			continue;
+		case '.':
+			periods++;
+			/* fallthru */
+		case ' ':
+			preceding_space_or_period = 1;
+			i++;
+			continue;
+		}
+		preceding_space_or_period = 0;
+		i++;
+	}
+}
+
 /*
  * Disable MSVCRT command line wildcard expansion (__getmainargs called from
  * mingw startup code, see init.c in mingw runtime).
diff --git a/compat/mingw.h b/compat/mingw.h
index e03aecfe2e..8c49c1d09b 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -429,6 +429,17 @@ int mingw_offset_1st_component(const char *path);
 #endif
 
 /**
+ * Verifies that the given path is a valid one on Windows.
+ *
+ * In particular, path segments are disallowed which end in a period or a
+ * space (except the special directories `.` and `..`).
+ *
+ * Returns 1 upon success, otherwise 0.
+ */
+int is_valid_win32_path(const char *path);
+#define is_valid_path(path) is_valid_win32_path(path)
+
+/**
  * Converts UTF-8 encoded string to UTF-16LE.
  *
  * To support repositories with legacy-encoded file names, invalid UTF-8 bytes