From: Simon Ruderich <simon@ruderich.org>
Date: Sun, 4 Apr 2021 06:25:57 +0000 (+0200)
Subject: fixperms: don't follow symlinks
X-Git-Url: https://ruderich.org/simon/gitweb/?a=commitdiff_plain;h=b3947ca010d5e49ae519722393dd4d2d2a3c19f7;p=safcm%2Fsafcm.git

fixperms: don't follow symlinks

This prevents a possible TOCTOU when chmodding files.
---

diff --git a/cmd/safcm/fixperms.go b/cmd/safcm/fixperms.go
index 6770934..47e4fb9 100644
--- a/cmd/safcm/fixperms.go
+++ b/cmd/safcm/fixperms.go
@@ -23,6 +23,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"syscall"
 
 	"ruderich.org/simon/safcm/cmd/safcm/config"
 )
@@ -78,7 +79,7 @@ func fixpermsWalkDirFunc(path string, d fs.DirEntry, err error) error {
 			// This is safe because perm does not include
 			// setuid/setgid/sticky which use different values in
 			// FileMode.
-			err := os.Chmod(path, fs.FileMode(perm))
+			err := chmodNoFollow(path, fs.FileMode(perm))
 			if err != nil {
 				return err
 			}
@@ -87,7 +88,7 @@ func fixpermsWalkDirFunc(path string, d fs.DirEntry, err error) error {
 		if perm != 0755 {
 			perm = 0755
 			log.Printf("chmodding %q to %#o", path, perm)
-			err := os.Chmod(path, fs.FileMode(perm))
+			err := chmodNoFollow(path, fs.FileMode(perm))
 			if err != nil {
 				return err
 			}
@@ -97,3 +98,20 @@ func fixpermsWalkDirFunc(path string, d fs.DirEntry, err error) error {
 
 	return nil
 }
+
+// chmodNoFollow works like os.Chmod but doesn't follow symlinks.
+func chmodNoFollow(path string, mode fs.FileMode) error {
+	x, err := os.OpenFile(path,
+		os.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, 0)
+	if err != nil {
+		return err
+	}
+	defer x.Close()
+
+	err = x.Chmod(mode)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}