fusefrontend: -allow_other: Use OpenatUser in Create FUSE call.

Revert commit b22cc03c7516b2003880db8375d26c76d6dff093.

Instead of manually adjusting the user and mode after creating the
file, adjust effective permissions and let the kernel deal with it.

Related to https://github.com/rfjakob/gocryptfs/issues/338.

1 files changed, 6 insertions, 28 deletions

diff --git a/internal/fusefrontend/fs.go b/internal/fusefrontend/fs.go
index 2cd4630..2c6ac5a 100644
--- a/internal/fusefrontend/fs.go
+++ b/internal/fusefrontend/fs.go
@@ -238,15 +238,11 @@ func (fs *FS) Create(path string, flags uint32, mode uint32, context *fuse.Conte
 		return nil, fuse.ToStatus(err)
 	}
 	defer syscall.Close(dirfd)
-	// Don't set full mode before we have set the correct owner. Files with SUID/SGID
-	// mode belonging to the wrong owner would be a security risk. Even for other
-	// modes, we don't want anyone else to open the file in the meantime: the fd would
-	// stay open and could later be used to read the file.
-	origMode := mode
-	if fs.args.PreserveOwner {
-		mode = 0000
-	}
 	fd := -1
+	// Make sure context is nil if we don't want to preserve the owner
+	if !fs.args.PreserveOwner {
+		context = nil
+	}
 	// Handle long file name
 	if !fs.args.PlaintextNames && nametransform.IsLongContent(cName) {
 		// Create ".name"
@@ -255,14 +251,14 @@ func (fs *FS) Create(path string, flags uint32, mode uint32, context *fuse.Conte
 			return nil, fuse.ToStatus(err)
 		}
 		// Create content
-		fd, err = syscallcompat.Openat(dirfd, cName, newFlags|os.O_CREATE|os.O_EXCL, mode)
+		fd, err = syscallcompat.OpenatUser(dirfd, cName, newFlags|os.O_CREATE|os.O_EXCL, mode, context)
 		if err != nil {
 			nametransform.DeleteLongNameAt(dirfd, cName)
 			return nil, fuse.ToStatus(err)
 		}
 	} else {
 		// Create content, normal (short) file name
-		fd, err = syscallcompat.Openat(dirfd, cName, newFlags|syscall.O_CREAT|syscall.O_EXCL, mode)
+		fd, err = syscallcompat.OpenatUser(dirfd, cName, newFlags|syscall.O_CREAT|syscall.O_EXCL, mode, context)
 		if err != nil {
 			// xfstests generic/488 triggers this
 			if err == syscall.EMFILE {
@@ -273,24 +269,6 @@ func (fs *FS) Create(path string, flags uint32, mode uint32, context *fuse.Conte
 			return nil, fuse.ToStatus(err)
 		}
 	}
-	// Set owner
-	if fs.args.PreserveOwner {
-		err = syscall.Fchown(fd, int(context.Owner.Uid), int(context.Owner.Gid))
-		if err != nil {
-			tlog.Warn.Printf("Create %q: Fchown %d:%d failed: %v", cName, context.Owner.Uid, context.Owner.Gid, err)
-			// In case of a failure, we don't want to proceed setting more
-			// permissive modes.
-			syscall.Close(fd)
-			return nil, fuse.ToStatus(err)
-		}
-	}
-	// Set mode
-	if mode != origMode {
-		err = syscall.Fchmod(fd, origMode)
-		if err != nil {
-			tlog.Warn.Printf("Create %q: Fchmod %#o -> %#o failed: %v", cName, mode, origMode, err)
-		}
-	}
 	f := os.NewFile(uintptr(fd), cName)
 	return NewFile(f, fs)
 }