Fix possible deadlock in UDP.Write().

In UDP endpoint.Write() sendUDP is called with e.mu Rlocked. But if this happens
to send a datagram over loopback which ends up generating an ICMP response of
say ErrNoPortReachable, the handling of the response in HandleControlPacket also
acquires e.mu using RLock. This is mostly fine unless there is a competing
caller trying to acquire e.mu in exclusive mode using Lock(). This will deadlock
as a caller waiting in Lock() disallows an new RLocks() to ensure it can
actually acquire the Lock.

This is documented here https://golang.org/pkg/sync/#RWMutex.

This change releases the endpoint mutex before calling sendUDP to resolve the
possibility of the deadlock.

Reported-by: syzbot+537989797548c66e8ee3@syzkaller.appspotmail.com
Reported-by: syzbot+eb0b73b4ab486f7673ba@syzkaller.appspotmail.com
PiperOrigin-RevId: 342894148

3 files changed, 73 insertions, 7 deletions

diff --git a/pkg/tcpip/transport/udp/BUILD b/pkg/tcpip/transport/udp/BUILD
index c78549424..7ebae63d8 100644
--- a/pkg/tcpip/transport/udp/BUILD
+++ b/pkg/tcpip/transport/udp/BUILD
@@ -56,6 +56,7 @@ go_test(
         "//pkg/tcpip/network/ipv4",
         "//pkg/tcpip/network/ipv6",
         "//pkg/tcpip/stack",
+        "//pkg/tcpip/transport/icmp",
         "//pkg/waiter",
     ],
 )
diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index 57976d4e3..835dcc54e 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -429,7 +429,13 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 	to := opts.To
 
 	e.mu.RLock()
-	defer e.mu.RUnlock()
+	lockReleased := false
+	defer func() {
+		if lockReleased {
+			return
+		}
+		e.mu.RUnlock()
+	}()
 
 	// If we've shutdown with SHUT_WR we are in an invalid state for sending.
 	if e.shutdownFlags&tcpip.ShutdownWrite != 0 {
@@ -475,7 +481,7 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 			if e.state != StateConnected {
 				err = tcpip.ErrInvalidEndpointState
 			}
-			return
+			return ch, err
 		}
 	} else {
 		// Reject destination address if it goes through a different
@@ -541,7 +547,24 @@ func (e *endpoint) write(p tcpip.Payloader, opts tcpip.WriteOptions) (int64, <-c
 		useDefaultTTL = false
 	}
 
-	if err := sendUDP(route, buffer.View(v).ToVectorisedView(), e.ID.LocalPort, dstPort, ttl, useDefaultTTL, e.sendTOS, e.owner, e.noChecksum); err != nil {
+	localPort := e.ID.LocalPort
+	sendTOS := e.sendTOS
+	owner := e.owner
+	noChecksum := e.noChecksum
+	lockReleased = true
+	e.mu.RUnlock()
+
+	// Do not hold lock when sending as loopback is synchronous and if the UDP
+	// datagram ends up generating an ICMP response then it can result in a
+	// deadlock where the ICMP response handling ends up acquiring this endpoint's
+	// mutex using e.mu.RLock() in endpoint.HandleControlPacket which can cause a
+	// deadlock if another caller is trying to acquire e.mu in exclusive mode w/
+	// e.mu.Lock(). Since e.mu.Lock() prevents any new read locks to ensure the
+	// lock can be eventually acquired.
+	//
+	// See: https://golang.org/pkg/sync/#RWMutex for details on why recursive read
+	// locking is prohibited.
+	if err := sendUDP(route, buffer.View(v).ToVectorisedView(), localPort, dstPort, ttl, useDefaultTTL, sendTOS, owner, noChecksum); err != nil {
 		return 0, nil, err
 	}
 	return int64(len(v)), nil, nil
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 764ad0857..492e277a8 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -32,6 +32,7 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/network/ipv4"
 	"gvisor.dev/gvisor/pkg/tcpip/network/ipv6"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/icmp"
 	"gvisor.dev/gvisor/pkg/tcpip/transport/udp"
 	"gvisor.dev/gvisor/pkg/waiter"
 )
@@ -54,6 +55,7 @@ const (
 	stackPort       = 1234
 	testAddr        = "\x0a\x00\x00\x02"
 	testPort        = 4096
+	invalidPort     = 8192
 	multicastAddr   = "\xe8\x2b\xd3\xea"
 	multicastV6Addr = "\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
 	broadcastAddr   = header.IPv4Broadcast
@@ -295,7 +297,8 @@ func newDualTestContext(t *testing.T, mtu uint32) *testContext {
 	t.Helper()
 	return newDualTestContextWithOptions(t, mtu, stack.Options{
 		NetworkProtocols:   []stack.NetworkProtocolFactory{ipv4.NewProtocol, ipv6.NewProtocol},
-		TransportProtocols: []stack.TransportProtocolFactory{udp.NewProtocol},
+		TransportProtocols: []stack.TransportProtocolFactory{udp.NewProtocol, icmp.NewProtocol6, icmp.NewProtocol4},
+		HandleLocal:        true,
 	})
 }
 
@@ -972,7 +975,7 @@ func testFailingWrite(c *testContext, flow testFlow, wantErr *tcpip.Error) {
 // provided.
 func testWrite(c *testContext, flow testFlow, checkers ...checker.NetworkChecker) uint16 {
 	c.t.Helper()
-	return testWriteInternal(c, flow, true, checkers...)
+	return testWriteAndVerifyInternal(c, flow, true, checkers...)
 }
 
 // testWriteWithoutDestination sends a packet of the given test flow from the
@@ -981,10 +984,10 @@ func testWrite(c *testContext, flow testFlow, checkers ...checker.NetworkChecker
 // checker functions provided.
 func testWriteWithoutDestination(c *testContext, flow testFlow, checkers ...checker.NetworkChecker) uint16 {
 	c.t.Helper()
-	return testWriteInternal(c, flow, false, checkers...)
+	return testWriteAndVerifyInternal(c, flow, false, checkers...)
 }
 
-func testWriteInternal(c *testContext, flow testFlow, setDest bool, checkers ...checker.NetworkChecker) uint16 {
+func testWriteNoVerify(c *testContext, flow testFlow, setDest bool) buffer.View {
 	c.t.Helper()
 	// Take a snapshot of the stats to validate them at the end of the test.
 	epstats := c.ep.Stats().(*tcpip.TransportEndpointStats).Clone()
@@ -1006,6 +1009,12 @@ func testWriteInternal(c *testContext, flow testFlow, setDest bool, checkers ...
 		c.t.Fatalf("Bad number of bytes written: got %v, want %v", n, len(payload))
 	}
 	c.checkEndpointWriteStats(1, epstats, err)
+	return payload
+}
+
+func testWriteAndVerifyInternal(c *testContext, flow testFlow, setDest bool, checkers ...checker.NetworkChecker) uint16 {
+	c.t.Helper()
+	payload := testWriteNoVerify(c, flow, setDest)
 	// Received the packet and check the payload.
 	b := c.getPacketAndVerify(flow, checkers...)
 	var udp header.UDP
@@ -1150,6 +1159,39 @@ func TestV4WriteOnConnected(t *testing.T) {
 	testWriteWithoutDestination(c, unicastV4)
 }
 
+func TestWriteOnConnectedInvalidPort(t *testing.T) {
+	protocols := map[string]tcpip.NetworkProtocolNumber{
+		"ipv4": ipv4.ProtocolNumber,
+		"ipv6": ipv6.ProtocolNumber,
+	}
+	for name, pn := range protocols {
+		t.Run(name, func(t *testing.T) {
+			c := newDualTestContext(t, defaultMTU)
+			defer c.cleanup()
+
+			c.createEndpoint(pn)
+			if err := c.ep.Connect(tcpip.FullAddress{Addr: stackAddr, Port: invalidPort}); err != nil {
+				c.t.Fatalf("Connect failed: %s", err)
+			}
+			writeOpts := tcpip.WriteOptions{
+				To: &tcpip.FullAddress{Addr: stackAddr, Port: invalidPort},
+			}
+			payload := buffer.View(newPayload())
+			n, _, err := c.ep.Write(tcpip.SlicePayload(payload), writeOpts)
+			if err != nil {
+				c.t.Fatalf("c.ep.Write(...) = %+s, want nil", err)
+			}
+			if got, want := n, int64(len(payload)); got != want {
+				c.t.Fatalf("c.ep.Write(...) wrote %d bytes, want %d bytes", got, want)
+			}
+
+			if err := c.ep.LastError(); err != tcpip.ErrConnectionRefused {
+				c.t.Fatalf("expected c.ep.LastError() == ErrConnectionRefused, got: %+v", err)
+			}
+		})
+	}
+}
+
 // TestWriteOnBoundToV4Multicast checks that we can send packets out of a socket
 // that is bound to a V4 multicast address.
 func TestWriteOnBoundToV4Multicast(t *testing.T) {