syscallcompat: add OpenNofollow helper

OpenNofollow = symlink-race-safe Open

Prepares fixing https://github.com/rfjakob/gocryptfs/issues/165

1 files changed, 49 insertions, 0 deletions

diff --git a/internal/syscallcompat/open_nofollow.go b/internal/syscallcompat/open_nofollow.go
new file mode 100644
index 0000000..c804e12
--- /dev/null
+++ b/internal/syscallcompat/open_nofollow.go
@@ -0,0 +1,49 @@
+package syscallcompat
+
+import (
+	"path/filepath"
+	"strings"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// OpenNofollow opens the file/dir at "relPath" in a way that is secure against
+// symlink attacks. Symlinks that are part of "relPath" are never followed.
+// This function is implemented by walking the directory tree, starting at
+// "baseDir", using the Openat syscall with the O_NOFOLLOW flag.
+// Symlinks that are part of the "baseDir" path are followed.
+func OpenNofollow(baseDir string, relPath string, flags int, mode uint32) (fd int, err error) {
+	if !filepath.IsAbs(baseDir) {
+		tlog.Warn.Printf("BUG: OpenNofollow called with relative baseDir=%q", baseDir)
+		return -1, syscall.EINVAL
+	}
+	if filepath.IsAbs(relPath) {
+		tlog.Warn.Printf("BUG: OpenNofollow called with absolute relPath=%q", relPath)
+		return -1, syscall.EINVAL
+	}
+	// Open the base dir
+	dirfd, err := syscall.Open(baseDir, syscall.O_RDONLY, 0)
+	if err != nil {
+		return -1, err
+	}
+	// Split the path into components and separate intermediate directories
+	// and the final basename
+	parts := strings.Split(relPath, "/")
+	dirs := parts[:len(parts)-1]
+	final := parts[len(parts)-1]
+	// Walk intermediate directories
+	var dirfd2 int
+	for _, name := range dirs {
+		dirfd2, err = Openat(dirfd, name, syscall.O_RDONLY|syscall.O_NOFOLLOW, 0)
+		syscall.Close(dirfd)
+		if err != nil {
+			return -1, err
+		}
+		dirfd = dirfd2
+	}
+	defer syscall.Close(dirfd)
+	// Open the final component with the flags and permissions requested by
+	// the user plus forced NOFOLLOW.
+	return Openat(dirfd, final, flags|syscall.O_NOFOLLOW, mode)
+}