Rpcinet is racy around shutdown flags.

Correct a data race in rpcinet where a shutdown and recvmsg can
race around shutown flags.

PiperOrigin-RevId: 201238366
Change-Id: I5eb06df4a2b4eba331eeb5de19076213081d581f

pkg/sentry/socket/rpcinet/socket.go

@@ -15,6 +15,7 @@
 package rpcinet
 
 import (
+	"sync/atomic"
 	"syscall"
 
 	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
@@ -57,7 +58,7 @@ type socketOperations struct {
 	// shState is the state of the connection with respect to shutdown. Because
 	// we're mixing non-blocking semantics on the other side we have to adapt for
 	// some strange differences between blocking and non-blocking sockets.
-	shState tcpip.ShutdownFlags
+	shState int32
 }
 
 // Verify that we actually implement socket.Socket.
@@ -105,26 +106,35 @@ func translateIOSyscallError(err error) error {
 // setShutdownFlags will set the shutdown flag so we can handle blocking reads
 // after a read shutdown.
 func (s *socketOperations) setShutdownFlags(how int) {
+	var f tcpip.ShutdownFlags
 	switch how {
 	case linux.SHUT_RD:
-		s.shState |= tcpip.ShutdownRead
+		f = tcpip.ShutdownRead
 	case linux.SHUT_WR:
-		s.shState |= tcpip.ShutdownWrite
+		f = tcpip.ShutdownWrite
 	case linux.SHUT_RDWR:
-		s.shState |= tcpip.ShutdownWrite | tcpip.ShutdownRead
+		f = tcpip.ShutdownWrite | tcpip.ShutdownRead
+	}
+
+	// Atomically update the flags.
+	for {
+		old := atomic.LoadInt32(&s.shState)
+		if atomic.CompareAndSwapInt32(&s.shState, old, old|int32(f)) {
+			break
+		}
 	}
 }
 
 func (s *socketOperations) resetShutdownFlags() {
-	s.shState = 0
+	atomic.StoreInt32(&s.shState, 0)
 }
 
 func (s *socketOperations) isShutRdSet() bool {
-	return s.shState&tcpip.ShutdownRead != 0
+	return atomic.LoadInt32(&s.shState)&int32(tcpip.ShutdownRead) != 0
 }
 
 func (s *socketOperations) isShutWrSet() bool {
-	return s.shState&tcpip.ShutdownWrite != 0
+	return atomic.LoadInt32(&s.shState)&int32(tcpip.ShutdownWrite) != 0
 }
 
 // Release implements fs.FileOperations.Release.