Fix write hang bug found by syzkaller.

After this change e.mu is only promoted to exclusively locked during
route.Resolve. It downgrades back to read-lock afterwards.
This prevents the second RLock() call gets stuck later in the stack.

https://syzkaller.appspot.com/bug?id=065b893bd8d1d04a4e0a1d53c578537cde1efe99

Syzkaller logs does not contain interesting stack traces.
The following stack trace is obtained by running repro locally.

goroutine 53 [semacquire, 3 minutes]:
runtime.gopark(0xfd4278, 0x1896320, 0xc000301912, 0x4)
        GOROOT/src/runtime/proc.go:304 +0xe0 fp=0xc0000e25f8 sp=0xc0000e25d8 pc=0x437170
runtime.goparkunlock(...)
        GOROOT/src/runtime/proc.go:310
runtime.semacquire1(0xc0001220b0, 0xc00000a300, 0x1, 0x0)
        GOROOT/src/runtime/sema.go:144 +0x1c0 fp=0xc0000e2660 sp=0xc0000e25f8 pc=0x4484e0
sync.runtime_Semacquire(0xc0001220b0)
        GOROOT/src/runtime/sema.go:56 +0x42 fp=0xc0000e2690 sp=0xc0000e2660 pc=0x448132
gvisor.dev/gvisor/pkg/sync.(*RWMutex).RLock(...)
        pkg/sync/rwmutex_unsafe.go:76
gvisor.dev/gvisor/pkg/tcpip/transport/udp.(*endpoint).HandleControlPacket(0xc000122000, 0x7ee5, 0xc00053c16c, 0x4, 0x5e21, 0xc00053c224, 0x4, 0x1, 0x0, 0xc00007ed00)
        pkg/tcpip/transport/udp/endpoint.go:1345 +0x169 fp=0xc0000e26d8 sp=0xc0000e2690 pc=0x9843f9
......
gvisor.dev/gvisor/pkg/tcpip/transport/udp.(*protocol).HandleUnknownDestinationPacket(0x18bb5a0, 0xc000556540, 0x5e21, 0xc00053c16c, 0x4, 0x7ee5, 0xc00053c1ec, 0x4, 0xc00007e680, 0x4)
        pkg/tcpip/transport/udp/protocol.go:143 +0xb9a fp=0xc0000e8260 sp=0xc0000e7510 pc=0x9859ba
......
gvisor.dev/gvisor/pkg/tcpip/transport/udp.sendUDP(0xc0001220d0, 0xc00053ece0, 0x1, 0x1, 0x883, 0x1405e217ee5, 0x11100a0, 0xc000592000, 0xf88780)
        pkg/tcpip/transport/udp/endpoint.go:924 +0x3b0 fp=0xc0000ed390 sp=0xc0000ec750 pc=0x981af0
gvisor.dev/gvisor/pkg/tcpip/transport/udp.(*endpoint).write(0xc000122000, 0x11104e0, 0xc00020a460, 0x0, 0x0, 0x0, 0x0, 0x0)
        pkg/tcpip/transport/udp/endpoint.go:510 +0x4ad fp=0xc0000ed658 sp=0xc0000ed390 pc=0x97f2dd

PiperOrigin-RevId: 315590041

1 files changed, 20 insertions, 9 deletions

diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index 8c7895713..c5e3c73ef 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -15,6 +15,7 @@
 package udp
 
 import (
+	"gvisor.dev/gvisor/pkg/sleep"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
@@ -425,24 +426,33 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 	}
 
 	var route *stack.Route
+	var resolve func(waker *sleep.Waker) (ch <-chan struct{}, err *tcpip.Error)
 	var dstPort uint16
 	if to == nil {
 		route = &e.route
 		dstPort = e.dstPort
-
-		if route.IsResolutionRequired() {
-			// Promote lock to exclusive if using a shared route, given that it may need to
-			// change in Route.Resolve() call below.
+		resolve = func(waker *sleep.Waker) (ch <-chan struct{}, err *tcpip.Error) {
+			// Promote lock to exclusive if using a shared route, given that it may
+			// need to change in Route.Resolve() call below.
 			e.mu.RUnlock()
-			defer e.mu.RLock()
-
 			e.mu.Lock()
-			defer e.mu.Unlock()
 
 			// Recheck state after lock was re-acquired.
 			if e.state != StateConnected {
-				return 0, nil, tcpip.ErrInvalidEndpointState
+				err = tcpip.ErrInvalidEndpointState
+			}
+			if err == nil && route.IsResolutionRequired() {
+				ch, err = route.Resolve(waker)
+			}
+
+			e.mu.Unlock()
+			e.mu.RLock()
+
+			// Recheck state after lock was re-acquired.
+			if e.state != StateConnected {
+				err = tcpip.ErrInvalidEndpointState
 			}
+			return
 		}
 	} else {
 		// Reject destination address if it goes through a different
@@ -473,10 +483,11 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 
 		route = &r
 		dstPort = dst.Port
+		resolve = route.Resolve
 	}
 
 	if route.IsResolutionRequired() {
-		if ch, err := route.Resolve(nil); err != nil {
+		if ch, err := resolve(nil); err != nil {
 			if err == tcpip.ErrWouldBlock {
 				return 0, ch, tcpip.ErrNoLinkAddress
 			}