ssh-tpm-agent: Add ssh-agent proxy functionality with -A

Allows `ssh-tpm-agent` act as a proxy between multiple ssh-agent
compatible agents.

It will deliver all available signers, from itself and all proxies, to
the ssh server, and then pass the signing requests to all the agents.

Fixes: #9

Signed-off-by: Morten Linderud <morten@linderud.pw>

agent/agent.go

@@ -33,6 +33,7 @@ type Agent struct {
 	quit     chan interface{}
 	wg       sync.WaitGroup
 	keys     map[string]*key.Key
+	agents   []agent.ExtendedAgent
 }
 
 var _ agent.ExtendedAgent = &Agent{}
@@ -64,6 +65,16 @@ func (a *Agent) Close() error {
 
 func (a *Agent) signers() ([]ssh.Signer, error) {
 	var signers []ssh.Signer
+
+	for _, agent := range a.agents {
+		l, err := agent.Signers()
+		if err != nil {
+			log.Printf("failed getting Signers from agent: %f", err)
+			continue
+		}
+		signers = append(signers, l...)
+	}
+
 	for _, k := range a.keys {
 		s, err := ssh.NewSignerFromSigner(signer.NewTPMSigner(k, a.tpm, a.pin))
 		if err != nil {
@@ -86,6 +97,15 @@ func (a *Agent) List() ([]*agent.Key, error) {
 	a.mu.Lock()
 	defer a.mu.Unlock()
 
+	for _, agent := range a.agents {
+		l, err := agent.List()
+		if err != nil {
+			log.Printf("failed getting list from agent: %v", err)
+			continue
+		}
+		agentKeys = append(agentKeys, l...)
+	}
+
 	for _, k := range a.keys {
 		pk, err := k.SSHPublicKey()
 		if err != nil {
@@ -115,6 +135,21 @@ func (a *Agent) SignWithFlags(key ssh.PublicKey, data []byte, flags agent.Signat
 		return s.(ssh.AlgorithmSigner).SignWithAlgorithm(rand.Reader, data, key.Type())
 	}
 
+	log.Printf("trying to sign as proxy...")
+	for _, agent := range a.agents {
+		signers, err := agent.Signers()
+		if err != nil {
+			log.Printf("failed getting signers from agent: %v", err)
+			continue
+		}
+		for _, s := range signers {
+			if !bytes.Equal(s.PublicKey().Marshal(), key.Marshal()) {
+				continue
+			}
+			return s.(ssh.AlgorithmSigner).SignWithAlgorithm(rand.Reader, data, key.Type())
+		}
+	}
+
 	return nil, fmt.Errorf("no private keys match the requested public key")
 }
 
@@ -230,12 +265,13 @@ func LoadKeys() (map[string]*key.Key, error) {
 	return keys, nil
 }
 
-func NewAgent(socketPath string, tpmFetch func() transport.TPMCloser, pin func(*key.Key) ([]byte, error)) *Agent {
+func NewAgent(socketPath string, agents []agent.ExtendedAgent, tpmFetch func() transport.TPMCloser, pin func(*key.Key) ([]byte, error)) *Agent {
 	a := &Agent{
-		tpm:  tpmFetch,
-		pin:  pin,
-		quit: make(chan interface{}),
-		keys: make(map[string]*key.Key),
+		agents: agents,
+		tpm:    tpmFetch,
+		pin:    pin,
+		quit:   make(chan interface{}),
+		keys:   make(map[string]*key.Key),
 	}
 	l, err := net.Listen("unix", socketPath)
 	if err != nil {

cmd/ssh-tpm-agent/main.go

@@ -5,14 +5,21 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"net"
 	"os"
+	"os/signal"
 	"path"
+	"path/filepath"
+	"syscall"
 
 	"github.com/foxboron/ssh-tpm-agent/agent"
 	"github.com/foxboron/ssh-tpm-agent/key"
 	"github.com/foxboron/ssh-tpm-agent/pinentry"
 	"github.com/foxboron/ssh-tpm-agent/utils"
 	"github.com/google/go-tpm/tpm2/transport"
+	sshagent "golang.org/x/crypto/ssh/agent"
+	"golang.org/x/exp/slices"
+	"golang.org/x/term"
 )
 
 var Version string
@@ -21,10 +28,12 @@ const usage = `Usage:
     ssh-tpm-agent -l [PATH]
 
 Options:
-    -l                path of the UNIX socket to listen on, defaults to
-                      $XDG_RUNTIME_DIR/ssh-tpm-agent.sock
+    -l PATH           Path of the UNIX socket to open, defaults to
+                      $XDG_RUNTIME_DIR/ssh-tpm-agent.sock.
 
-    --print-socket    prints the socket to STDIN
+    -A PATH           Fallback ssh-agent sockets for additional key lookup.
+
+    --print-socket    Prints the socket to STDIN.
 
 ssh-tpm-agent is a program that loads TPM sealed keys for public key
 authentication. It is an ssh-agent(1) compatible program and can be used for
@@ -43,6 +52,31 @@ Example:
     $ export SSH_AUTH_SOCK=$(ssh-tpm-agent --print-socket)
     $ ssh git@github.com`
 
+type SocketSet struct {
+	Value []string
+}
+
+func (s SocketSet) String() string {
+	return "set"
+}
+
+func (s *SocketSet) Set(p string) error {
+	if !slices.Contains(s.Value, p) {
+		s.Value = append(s.Value, p)
+	}
+	return nil
+}
+
+func (s SocketSet) Type() string {
+	return "[PATH]"
+}
+
+func NewSocketSet(allowed []string, d string) *SocketSet {
+	return &SocketSet{
+		Value: []string{},
+	}
+}
+
 func main() {
 	flag.Usage = func() {
 		fmt.Println(usage)
@@ -62,7 +96,10 @@ func main() {
 		return path.Join(dir, "ssh-tpm-agent.sock")
 	}()
 
+	var sockets SocketSet
+
 	flag.StringVar(&socketPath, "l", defaultSocketPath, "path of the UNIX socket to listen on")
+	flag.Var(&sockets, "A", "fallback ssh-agent sockets")
 	flag.BoolVar(&swtpmFlag, "swtpm", false, "use swtpm instead of actual tpm")
 	flag.BoolVar(&printSocketFlag, "print-socket", false, "print path of UNIX socket to stdout")
 	flag.Parse()
@@ -89,7 +126,18 @@ func main() {
 	}
 	log.Printf("Listening on %v\n", socketPath)
 
+	var agents []sshagent.ExtendedAgent
+
+	for _, s := range sockets.Value {
+		conn, err := net.Dial("unix", s)
+		if err != nil {
+			log.Fatal(err)
+		}
+		agents = append(agents, sshagent.NewClient(conn))
+	}
+
 	a := agent.NewAgent(socketPath,
+		agents,
 		// TPM Callback
 		func() (tpm transport.TPMCloser) {
 			// the agent will close the TPM after this is called

cmd/ssh-tpm-agent/main_test.go

@@ -17,6 +17,7 @@ import (
 	"github.com/google/go-tpm/tpm2/transport"
 	"github.com/google/go-tpm/tpm2/transport/simulator"
 	"golang.org/x/crypto/ssh"
+	sshagent "golang.org/x/crypto/ssh/agent"
 )
 
 func newSSHKey() ssh.Signer {
@@ -127,6 +128,7 @@ func TestSSHAuth(t *testing.T) {
 	socket := path.Join(t.TempDir(), "socket")
 
 	ag := agent.NewAgent(socket,
+		[]sshagent.ExtendedAgent{},
 		// TPM Callback
 		func() transport.TPMCloser {
 			return tpm

go.mod

@@ -15,5 +15,6 @@ require (
 	github.com/rs/zerolog v1.26.0 // indirect
 	go.uber.org/atomic v1.7.0 // indirect
 	go.uber.org/multierr v1.8.0 // indirect
+	golang.org/x/exp v0.0.0-20230801115018-d63ba01acd4b // indirect
 	golang.org/x/sys v0.10.0 // indirect
 )

go.sum

@@ -36,6 +36,8 @@ golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACk
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/crypto v0.11.0 h1:6Ewdq3tDic1mg5xRO4milcWCfMVQhI4NkqWWvqejpuA=
 golang.org/x/crypto v0.11.0/go.mod h1:xgJhtzW8F9jGdVFWZESrid1U1bjeNy4zgy5cRr/CIio=
+golang.org/x/exp v0.0.0-20230801115018-d63ba01acd4b h1:r+vk0EmXNmekl0S0BascoeeoHk/L7wmaW2QF90K+kYI=
+golang.org/x/exp v0.0.0-20230801115018-d63ba01acd4b/go.mod h1:FXUEEKJgO7OQYeo8N01OfiKP8RXMtf6e8aTskBGqWdc=
 golang.org/x/mod v0.4.2/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=