crypt: Fix data corruption caused by seeking - #828

The corruption was caused when the file was read to the end thus
setting io.EOF and returning the buffers to the pool.  Seek reset the
EOF and carried on using the buffers that had been returned to the
pool thus causing corruption when other goroutines claimed the buffers
simultaneously.

Fix by resetting the buffer pointers to nil when released and claiming
new ones when seek resets EOF.  Also added locking for Read and Seek
which shouldn't be run concurrently.

crypt/cipher.go

@@ -8,7 +8,6 @@ import (
 	"encoding/base32"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"strings"
 	"sync"
 	"unicode/utf8"
@@ -48,6 +47,7 @@ var (
 	ErrorBadBase32Encoding       = errors.New("bad base32 filename encoding")
 	ErrorFileClosed              = errors.New("file already closed")
 	ErrorNotAnEncryptedFile      = errors.New("not an encrypted file - no \"" + encryptedSuffix + "\" suffix")
+	ErrorBadSeek                 = errors.New("Seek beyond end of file")
 	defaultSalt                  = []byte{0xA8, 0x0D, 0xF4, 0x3A, 0x8F, 0xBD, 0x03, 0x08, 0xA7, 0xCA, 0xB8, 0x3E, 0x58, 0x1F, 0x86, 0xB1}
 )
 
@@ -404,6 +404,7 @@ func (n *nonce) add(x uint64) {
 
 // encrypter encrypts an io.Reader on the fly
 type encrypter struct {
+	mu       sync.Mutex
 	in       io.Reader
 	c        *cipher
 	nonce    nonce
@@ -437,6 +438,9 @@ func (c *cipher) newEncrypter(in io.Reader) (*encrypter, error) {
 
 // Read as per io.Reader
 func (fh *encrypter) Read(p []byte) (n int, err error) {
+	fh.mu.Lock()
+	defer fh.mu.Unlock()
+
 	if fh.err != nil {
 		return 0, fh.err
 	}
@@ -474,7 +478,9 @@ func (fh *encrypter) finish(err error) (int, error) {
 	}
 	fh.err = err
 	fh.c.putBlock(fh.buf)
+	fh.buf = nil
 	fh.c.putBlock(fh.readBuf)
+	fh.readBuf = nil
 	return 0, err
 }
 
@@ -489,6 +495,7 @@ func (c *cipher) EncryptData(in io.Reader) (io.Reader, error) {
 
 // decrypter decrypts an io.ReaderCloser on the fly
 type decrypter struct {
+	mu           sync.Mutex
 	rc           io.ReadCloser
 	nonce        nonce
 	initialNonce nonce
@@ -551,37 +558,48 @@ func (c *cipher) newDecrypterSeek(open OpenAtOffset, offset int64) (fh *decrypte
 	return fh, nil
 }
 
-// Read as per io.Reader
-func (fh *decrypter) Read(p []byte) (n int, err error) {
-	if fh.err != nil {
-		return 0, fh.err
-	}
-	if fh.bufIndex >= fh.bufSize {
-		// Read data
+// read data into internal buffer - call with fh.mu held
+func (fh *decrypter) fillBuffer() (err error) {
 	// FIXME should overlap the reads with a go-routine and 2 buffers?
 	readBuf := fh.readBuf
-		n, err = io.ReadFull(fh.rc, readBuf)
+	n, err := io.ReadFull(fh.rc, readBuf)
 	if err == io.EOF {
 		// ReadFull only returns n=0 and EOF
-			return 0, fh.finish(io.EOF)
+		return io.EOF
 	} else if err == io.ErrUnexpectedEOF {
 		// Next read will return EOF
 	} else if err != nil {
-			return 0, fh.finish(err)
+		return err
 	}
 	// Check header + 1 byte exists
 	if n <= blockHeaderSize {
-			return 0, fh.finish(ErrorEncryptedFileBadHeader)
+		return ErrorEncryptedFileBadHeader
 	}
 	// Decrypt the block using the nonce
 	block := fh.buf
 	_, ok := secretbox.Open(block[:0], readBuf[:n], fh.nonce.pointer(), &fh.c.dataKey)
 	if !ok {
-			return 0, fh.finish(ErrorEncryptedBadBlock)
+		return ErrorEncryptedBadBlock
 	}
 	fh.bufIndex = 0
 	fh.bufSize = n - blockHeaderSize
 	fh.nonce.increment()
+	return nil
+}
+
+// Read as per io.Reader
+func (fh *decrypter) Read(p []byte) (n int, err error) {
+	fh.mu.Lock()
+	defer fh.mu.Unlock()
+
+	if fh.err != nil {
+		return 0, fh.err
+	}
+	if fh.bufIndex >= fh.bufSize {
+		err = fh.fillBuffer()
+		if err != nil {
+			return 0, fh.finish(err)
+		}
 	}
 	n = copy(p, fh.buf[fh.bufIndex:fh.bufSize])
 	fh.bufIndex += n
@@ -590,6 +608,9 @@ func (fh *decrypter) Read(p []byte) (n int, err error) {
 
 // Seek as per io.Seeker
 func (fh *decrypter) Seek(offset int64, whence int) (int64, error) {
+	fh.mu.Lock()
+	defer fh.mu.Unlock()
+
 	if fh.open == nil {
 		return 0, fh.finish(errors.New("can't seek - not initialised with newDecrypterSeek"))
 	}
@@ -599,7 +620,7 @@ func (fh *decrypter) Seek(offset int64, whence int) (int64, error) {
 
 	// Reset error or return it if not EOF
 	if fh.err == io.EOF {
-		fh.err = nil
+		fh.unFinish()
 	} else if fh.err != nil {
 		return 0, fh.err
 	}
@@ -636,16 +657,18 @@ func (fh *decrypter) Seek(offset int64, whence int) (int64, error) {
 		fh.rc = rc
 	}
 
-	// Empty the buffer
-	fh.bufIndex = 0
-	fh.bufSize = 0
-
-	// Discard excess bytes
-	_, err := io.CopyN(ioutil.Discard, fh, discard)
+	// Fill the buffer
+	err := fh.fillBuffer()
 	if err != nil {
 		return 0, fh.finish(err)
 	}
 
+	// Discard bytes from the buffer
+	if int(discard) > fh.bufSize {
+		return 0, fh.finish(ErrorBadSeek)
+	}
+	fh.bufIndex = int(discard)
+
 	return offset, nil
 }
 
@@ -656,12 +679,31 @@ func (fh *decrypter) finish(err error) error {
 	}
 	fh.err = err
 	fh.c.putBlock(fh.buf)
+	fh.buf = nil
 	fh.c.putBlock(fh.readBuf)
+	fh.readBuf = nil
 	return err
 }
 
+// unFinish undoes the effects of finish
+func (fh *decrypter) unFinish() {
+	// Clear error
+	fh.err = nil
+
+	// reinstate the buffers
+	fh.buf = fh.c.getBlock()
+	fh.readBuf = fh.c.getBlock()
+
+	// Empty the buffer
+	fh.bufIndex = 0
+	fh.bufSize = 0
+}
+
 // Close
 func (fh *decrypter) Close() error {
+	fh.mu.Lock()
+	defer fh.mu.Unlock()
+
 	// Check already closed
 	if fh.err == ErrorFileClosed {
 		return fh.err