hash/crc32: fix race between lazy Castagnoli init and Update/Write

The switch on tab is checking tab == castagnoliTable,
but castagnoliTable can change value during a concurrent
call to MakeTable.

Fixes #41911.

Change-Id: I6124dcdbf33e17fe302baa3e1aa03202dec61b4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/261639
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 20 insertions, 6 deletions

diff --git a/src/hash/crc32/crc32.go b/src/hash/crc32/crc32.go
index 908b84adcb..f330fdb77a 100644
--- a/src/hash/crc32/crc32.go
+++ b/src/hash/crc32/crc32.go
@@ -16,6 +16,7 @@ import (
 	"errors"
 	"hash"
 	"sync"
+	"sync/atomic"
 )
 
 // The size of a CRC-32 checksum in bytes.
@@ -78,6 +79,7 @@ var castagnoliTable8 *slicing8Table
 var castagnoliArchImpl bool
 var updateCastagnoli func(crc uint32, p []byte) uint32
 var castagnoliOnce sync.Once
+var haveCastagnoli uint32
 
 func castagnoliInit() {
 	castagnoliTable = simpleMakeTable(Castagnoli)
@@ -93,6 +95,8 @@ func castagnoliInit() {
 			return slicingUpdate(crc, castagnoliTable8, p)
 		}
 	}
+
+	atomic.StoreUint32(&haveCastagnoli, 1)
 }
 
 // IEEETable is the table for the IEEE polynomial.
@@ -208,10 +212,10 @@ func readUint32(b []byte) uint32 {
 
 // Update returns the result of adding the bytes in p to the crc.
 func Update(crc uint32, tab *Table, p []byte) uint32 {
-	switch tab {
-	case castagnoliTable:
+	switch {
+	case atomic.LoadUint32(&haveCastagnoli) != 0 && tab == castagnoliTable:
 		return updateCastagnoli(crc, p)
-	case IEEETable:
+	case tab == IEEETable:
 		// Unfortunately, because IEEETable is exported, IEEE may be used without a
 		// call to MakeTable. We have to make sure it gets initialized in that case.
 		ieeeOnce.Do(ieeeInit)
@@ -222,10 +226,10 @@ func Update(crc uint32, tab *Table, p []byte) uint32 {
 }
 
 func (d *digest) Write(p []byte) (n int, err error) {
-	switch d.tab {
-	case castagnoliTable:
+	switch {
+	case atomic.LoadUint32(&haveCastagnoli) != 0 && d.tab == castagnoliTable:
 		d.crc = updateCastagnoli(d.crc, p)
-	case IEEETable:
+	case d.tab == IEEETable:
 		// We only create digest objects through New() which takes care of
 		// initialization in this case.
 		d.crc = updateIEEE(d.crc, p)
diff --git a/src/hash/crc32/crc32_test.go b/src/hash/crc32/crc32_test.go
index 4bdafaf8f5..cbb869dfd6 100644
--- a/src/hash/crc32/crc32_test.go
+++ b/src/hash/crc32/crc32_test.go
@@ -13,6 +13,16 @@ import (
 	"testing"
 )
 
+// First test, so that it can be the one to initialize castagnoliTable.
+func TestCastagnoliRace(t *testing.T) {
+	// The MakeTable(Castagnoli) lazily initializes castagnoliTable,
+	// which races with the switch on tab during Write to check
+	// whether tab == castagnoliTable.
+	ieee := NewIEEE()
+	go MakeTable(Castagnoli)
+	ieee.Write([]byte("hello"))
+}
+
 type test struct {
 	ieee, castagnoli    uint32
 	in                  string