From e56c2e44cc4dd1484cae78956f8ff72a9eea42f2 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Fri, 2 Jun 2023 14:24:44 +0200
Subject: fusefrontend: sharedstorage: use byte-range lock on file header
 creation

Multiple host writing to the same empty file at the same time
could have overwritten each other's newly created file header,
leading to data corruption.

Fix the race by placing a byte-range lock on the file when
creating the file header.
---
 internal/fusefrontend/file.go      | 16 ++++++++++++++++
 internal/fusefrontend/file_lock.go | 29 +++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 internal/fusefrontend/file_lock.go

(limited to 'internal/fusefrontend')

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index 64c6ca0..40dce6d 100644
--- a/internal/fusefrontend/file.go
+++ b/internal/fusefrontend/file.go
@@ -14,6 +14,8 @@ import (
 	"sync"
 	"syscall"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/hanwen/go-fuse/v2/fs"
 	"github.com/hanwen/go-fuse/v2/fuse"
 
@@ -90,6 +92,13 @@ func (f *File) readFileID() ([]byte, error) {
 	// and not only the header. A header-only file will be considered empty.
 	// This makes File ID poisoning more difficult.
 	readLen := contentenc.HeaderLen + 1
+	if f.rootNode.args.SharedStorage {
+		// With -sharedstorage, we consider a header-only file as valid, because
+		// another gocryptfs process may have either:
+		// 1) just created the header, and not written further data yet.
+		// 2) truncated the file down to just the header.
+		readLen = contentenc.HeaderLen
+	}
 	buf := make([]byte, readLen)
 	n, err := f.fd.ReadAt(buf, 0)
 	if err != nil {
@@ -266,18 +275,25 @@ func (f *File) doWrite(data []byte, off int64) (uint32, syscall.Errno) {
 	//
 	// If the file ID is not cached, read it from disk
 	if f.fileTableEntry.ID == nil {
+		if err := f.LockSharedStorage(unix.F_WRLCK, 0, contentenc.HeaderLen); err != nil {
+			return 0, fs.ToErrno(err)
+		}
 		var err error
 		fileID, err := f.readFileID()
 		// Write a new file header if the file is empty
 		if err == io.EOF {
 			fileID, err = f.createHeader()
 			fileWasEmpty = true
+			// Having the unlock three times is ugly. But every other way I tried is even uglier.
+			f.LockSharedStorage(unix.F_UNLCK, 0, contentenc.HeaderLen)
 		} else if err != nil {
 			// Other errors mean readFileID() found a corrupt header
 			tlog.Warn.Printf("doWrite %d: corrupt header: %v", f.qIno.Ino, err)
+			f.LockSharedStorage(unix.F_UNLCK, 0, contentenc.HeaderLen)
 			return 0, syscall.EIO
 		}
 		if err != nil {
+			f.LockSharedStorage(unix.F_UNLCK, 0, contentenc.HeaderLen)
 			return 0, fs.ToErrno(err)
 		}
 		f.fileTableEntry.ID = fileID
diff --git a/internal/fusefrontend/file_lock.go b/internal/fusefrontend/file_lock.go
new file mode 100644
index 0000000..6f92cfe
--- /dev/null
+++ b/internal/fusefrontend/file_lock.go
@@ -0,0 +1,29 @@
+package fusefrontend
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/syscallcompat"
+)
+
+// SharedStorageLock conveniently wraps F_OFD_SETLKW
+// See https://man7.org/linux/man-pages/man2/fcntl.2.html -> "Open file description locks (non-POSIX)"
+//
+// lkType is one of:
+// * unix.F_RDLCK (shared read lock)
+// * unix.F_WRLCK (exclusive write lock)
+// * unix.F_UNLCK (unlock)
+//
+// This function is a no-op if args.SharedStorage == false.
+func (f *File) LockSharedStorage(lkType int16, lkStart int64, lkLen int64) error {
+	if !f.rootNode.args.SharedStorage {
+		return nil
+	}
+	lk := unix.Flock_t{
+		Type:   lkType,
+		Whence: unix.SEEK_SET,
+		Start:  lkStart,
+		Len:    lkLen,
+	}
+	return unix.FcntlFlock(uintptr(f.intFd()), syscallcompat.F_OFD_SETLKW, &lk)
+}
-- 
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