Extend ownership validation to entire directory structure

A previous commit extended file ownership validation to policy and
protector files (by default -- there's an opt-out in /etc/fscrypt.conf).

However, that didn't apply to the parent directories:

	MOUNTPOINT
	MOUNTPOINT/.fscrypt
	MOUNTPOINT/.fscrypt/policies
	MOUNTPOINT/.fscrypt/protectors

The problem is that if the parent directories aren't trusted (owned by
another non-root user), then untrusted changes to their contents can be
made at any time, including the introduction of symlinks and so on.

While it's debatable how much of a problem this really is, given the
other validations that are done, it seems to be appropriate to validate
the parent directories too.

Therefore, this commit applies the same ownership validations to the
above four directories as are done on the metadata files themselves.

In addition, it is validated that none of these directories are symlinks
except for ".fscrypt" where this is explicitly supported.

1 files changed, 10 insertions, 19 deletions

diff --git a/filesystem/path.go b/filesystem/path.go
index fa38701..8cfb235 100644
--- a/filesystem/path.go
+++ b/filesystem/path.go
@@ -38,9 +38,6 @@ func OpenFileOverridingUmask(name string, flag int, perm os.FileMode) (*os.File,
 	return os.OpenFile(name, flag, perm)
 }
 
-// We only check the unix permissions and the sticky bit
-const permMask = os.ModeSticky | os.ModePerm
-
 // canonicalizePath turns path into an absolute path without symlinks.
 func canonicalizePath(path string) (string, error) {
 	path, err := filepath.Abs(path)
@@ -67,28 +64,22 @@ func loggedStat(name string) (os.FileInfo, error) {
 	return info, err
 }
 
+// loggedLstat runs os.Lstat (doesn't dereference trailing symlink), but it logs
+// the error if lstat returns any error other than nil or IsNotExist.
+func loggedLstat(name string) (os.FileInfo, error) {
+	info, err := os.Lstat(name)
+	if err != nil && !os.IsNotExist(err) {
+		log.Print(err)
+	}
+	return info, err
+}
+
 // isDir returns true if the path exists and is that of a directory.
 func isDir(path string) bool {
 	info, err := loggedStat(path)
 	return err == nil && info.IsDir()
 }
 
-// isDirCheckPerm returns true if the path exists and is a directory. If the
-// specified permissions and sticky bit of mode do not match the path, an error
-// is logged.
-func isDirCheckPerm(path string, mode os.FileMode) bool {
-	info, err := loggedStat(path)
-	// Check if directory
-	if err != nil || !info.IsDir() {
-		return false
-	}
-	// Check for bad permissions
-	if info.Mode()&permMask != mode&permMask {
-		log.Printf("directory %s has incorrect permissions", path)
-	}
-	return true
-}
-
 // isRegularFile returns true if the path exists and is that of a regular file.
 func isRegularFile(path string) bool {
 	info, err := loggedStat(path)