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-rw-r--r--internal/syscallcompat/open_nofollow.go49
-rw-r--r--internal/syscallcompat/open_nofollow_test.go37
2 files changed, 86 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)
+}
diff --git a/internal/syscallcompat/open_nofollow_test.go b/internal/syscallcompat/open_nofollow_test.go
new file mode 100644
index 0000000..e9cdf77
--- /dev/null
+++ b/internal/syscallcompat/open_nofollow_test.go
@@ -0,0 +1,37 @@
+package syscallcompat
+
+import (
+ "os"
+ "syscall"
+ "testing"
+)
+
+func TestOpenNofollow(t *testing.T) {
+ err := os.MkdirAll(tmpDir+"/d1/d2/d3", 0700)
+ if err != nil {
+ t.Fatal(err)
+ }
+ // Create a file
+ fd, err := OpenNofollow(tmpDir, "d1/d2/d3/f1", syscall.O_RDWR|syscall.O_CREAT|syscall.O_EXCL, 0600)
+ if err != nil {
+ t.Fatal(err)
+ }
+ syscall.Close(fd)
+ _, err = os.Stat(tmpDir + "/d1/d2/d3/f1")
+ if err != nil {
+ t.Fatal(err)
+ }
+ // Replace "d1" with a symlink - open should fail with ELOOP
+ err = os.Rename(tmpDir+"/d1", tmpDir+"/d1.renamed")
+ if err != nil {
+ t.Fatal(err)
+ }
+ os.Symlink(tmpDir+"/d1.renamed", tmpDir+"/d1")
+ fd, err = OpenNofollow(tmpDir, "d1/d2/d3/f1", syscall.O_RDWR|syscall.O_CREAT, 0600)
+ if err == nil {
+ t.Fatalf("should have failed")
+ }
+ if err != syscall.ELOOP {
+ t.Errorf("expected ELOOP, got %v", err)
+ }
+}