ssh-agent: implement the clientside for agent forwarding auth.

This can only be used to authenticate the client, not to allow the
connected server to transfer agent requests

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/agent.h

@@ -71,6 +71,7 @@ struct ssh_agent_struct {
   struct ssh_socket_struct *sock;
   ssh_buffer ident;
   unsigned int count;
+  ssh_channel channel;
 };
 
 #ifndef _WIN32

include/libssh/libssh.h

@@ -521,6 +521,7 @@ LIBSSH_API int ssh_scp_write(ssh_scp scp, const void *buffer, size_t len);
 LIBSSH_API int ssh_select(ssh_channel *channels, ssh_channel *outchannels, socket_t maxfd,
     fd_set *readfds, struct timeval *timeout);
 LIBSSH_API int ssh_service_request(ssh_session session, const char *service);
+LIBSSH_API int ssh_set_agent_channel(ssh_session session, ssh_channel channel);
 LIBSSH_API void ssh_set_blocking(ssh_session session, int blocking);
 LIBSSH_API void ssh_set_fd_except(ssh_session session);
 LIBSSH_API void ssh_set_fd_toread(ssh_session session);

src/agent.c

@@ -83,13 +83,17 @@ static void agent_put_u32(void *vp, uint32_t v) {
   p[3] = (uint8_t)v & 0xff;
 }
 
-static size_t atomicio(ssh_socket s, void *buf, size_t n, int do_read) {
+static size_t atomicio(struct ssh_agent_struct *agent, void *buf, size_t n, int do_read) {
   char *b = buf;
   size_t pos = 0;
   ssize_t res;
   ssh_pollfd_t pfd;
-  socket_t fd = ssh_socket_get_fd_in(s);
+  ssh_channel channel = agent->channel;
+  socket_t fd;
 
+  /* Using a socket ? */
+  if (channel == NULL) {
+    fd = ssh_socket_get_fd_in(agent->sock);
     pfd.fd = fd;
     pfd.events = do_read ? POLLIN : POLLOUT;
 
@@ -121,8 +125,22 @@ static size_t atomicio(ssh_socket s, void *buf, size_t n, int do_read) {
         pos += (size_t) res;
         }
       }
-
       return pos;
+    } else {
+      /* using an SSH channel */
+      while (n > pos){
+        if (do_read)
+          res = ssh_channel_read(channel,b + pos, n-pos, 0);
+        else
+          res = ssh_channel_write(channel, b+pos, n-pos);
+        if (res == SSH_AGAIN)
+          continue;
+        if (res == SSH_ERROR)
+          return 0;
+        pos += (size_t)res;
+      }
+      return pos;
+    }
 }
 
 ssh_agent agent_new(struct ssh_session_struct *session) {
@@ -140,10 +158,34 @@ ssh_agent agent_new(struct ssh_session_struct *session) {
     SAFE_FREE(agent);
     return NULL;
   }
-
+  agent->channel = NULL;
   return agent;
 }
 
+static void agent_set_channel(struct ssh_agent_struct *agent, ssh_channel channel){
+  agent->channel = channel;
+}
+
+/** @brief sets the SSH agent channel.
+ * The SSH agent channel will be used to authenticate this client using
+ * an agent through a channel, from another session. The most likely use
+ * is to implement SSH Agent forwarding into a SSH proxy.
+ * @param[in] channel a SSH channel from another session.
+ * @returns SSH_OK in case of success
+ *          SSH_ERROR in case of an error
+ */
+int ssh_set_agent_channel(ssh_session session, ssh_channel channel){
+  if (!session)
+    return SSH_ERROR;
+  if (!session->agent){
+    ssh_set_error(session, SSH_REQUEST_DENIED, "Session has no active agent");
+    return SSH_ERROR;
+  }
+  agent_set_channel(session->agent, channel);
+  return SSH_OK;
+}
+
+
 void agent_close(struct ssh_agent_struct *agent) {
   if (agent == NULL) {
     return;
@@ -174,6 +216,9 @@ static int agent_connect(ssh_session session) {
     return -1;
   }
 
+  if (session->agent->channel != NULL)
+    return 0;
+
   auth_sock = getenv("SSH_AUTH_SOCK");
 
   if (auth_sock && *auth_sock) {
@@ -216,8 +261,8 @@ static int agent_talk(struct ssh_session_struct *session,
   agent_put_u32(payload, len);
 
   /* send length and then the request packet */
-  if (atomicio(session->agent->sock, payload, 4, 0) == 4) {
+  if (atomicio(session->agent, payload, 4, 0) == 4) {
-    if (atomicio(session->agent->sock, buffer_get_rest(request), len, 0)
+    if (atomicio(session->agent, buffer_get_rest(request), len, 0)
         != len) {
       SSH_LOG(session, SSH_LOG_WARN, "atomicio sending request failed: %s",
           strerror(errno));
@@ -231,7 +276,7 @@ static int agent_talk(struct ssh_session_struct *session,
   }
 
   /* wait for response, read the length of the response packet */
-  if (atomicio(session->agent->sock, payload, 4, 1) != 4) {
+  if (atomicio(session->agent, payload, 4, 1) != 4) {
     SSH_LOG(session, SSH_LOG_WARN, "atomicio read response length failed: %s",
         strerror(errno));
     return -1;
@@ -250,7 +295,7 @@ static int agent_talk(struct ssh_session_struct *session,
     if (n > sizeof(payload)) {
       n = sizeof(payload);
     }
-    if (atomicio(session->agent->sock, payload, n, 1) != n) {
+    if (atomicio(session->agent, payload, n, 1) != n) {
       SSH_LOG(session, SSH_LOG_WARN,
           "Error reading response from authentication socket.");
       return -1;