server: Wait until handshake complete before calling .open callback

Currently we call the plugin .open callback as soon as we receive a
TCP connection:

  $ nbdkit -fv --tls=require --tls-certificates=tests/pki null \
           --run "telnet localhost 10809"
  [...]
  Trying ::1...
  Connected to localhost.
  Escape character is '^]'.
  nbdkit: debug: accepted connection
  nbdkit: debug: null: open readonly=0       ◀ NOTE
  nbdkit: null[1]: debug: newstyle negotiation: flags: global 0x3
  NBDMAGICIHAVEOPT

In plugins such as curl, guestfs, ssh, vddk and others we do a
considerable amount of work in the .open callback (such as making a
remote connection or launching an appliance).  Therefore we are
providing an easy Denial of Service / Amplification Attack for
unauthorized clients to cause a lot of work to be done for only the
cost of a simple TCP 3 way handshake.

This commit moves the call to the .open callback after the NBD
handshake has mostly completed.  In particular TLS authentication must
be complete before we will call into the plugin.

It is unlikely that there are plugins which really depend on the
current behaviour of .open (which I found surprising even though I
guess I must have written it).  If there are then we could add a new
.connect callback or similar to allow plugins to get control at the
earlier point in the connection.

After this change you can see that the .open callback is not called
from just a simple TCP connection:

  $ ./nbdkit -fv --tls=require --tls-certificates=tests/pki null \
             --run "telnet localhost 10809"
  [...]
  Trying ::1...
  Connected to localhost.
  Escape character is '^]'.
  nbdkit: debug: accepted connection
  nbdkit: null[1]: debug: newstyle negotiation: flags: global 0x3
  NBDMAGICIHAVEOPT
  xx
  nbdkit: null[1]: debug: newstyle negotiation: client flags: 0xd0a7878
  nbdkit: null[1]: error: client requested unknown flags 0xd0a7878
  Connection closed by foreign host.
  nbdkit: debug: null: unload plugin

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
(cherry picked from commit c05686f)

server/connections.c

@@ -174,12 +174,6 @@ _handle_single_connection (int sockin, int sockout)
   if (!conn)
     goto done;
 
-  lock_request (conn);
-  r = backend->open (backend, conn, readonly);
-  unlock_request (conn);
-  if (r == -1)
-    goto done;
-
   /* NB: because of an asynchronous exit backend can be set to NULL at
    * just about any time.
    */
@@ -189,17 +183,11 @@ _handle_single_connection (int sockin, int sockout)
     plugin_name = "(unknown)";
   threadlocal_set_name (plugin_name);
 
-  /* Prepare (for filters), called just after open. */
-  lock_request (conn);
-  if (backend)
-    r = backend->prepare (backend, conn);
-  else
-    r = 0;
-  unlock_request (conn);
-  if (r == -1)
-    goto done;
-
-  /* Handshake. */
+  /* NBD handshake.
+   *
+   * Note that this calls the backend .open callback when it is safe
+   * to do so (eg. after TLS authentication).
+   */
   if (protocol_handshake (conn) == -1)
     goto done;
 

server/internal.h

@@ -198,8 +198,9 @@ extern int connection_set_status (struct connection *conn, int value)
 /* protocol-handshake.c */
 extern int protocol_handshake (struct connection *conn)
   __attribute__((__nonnull__ (1)));
-extern int protocol_compute_eflags (struct connection *conn, uint16_t *flags)
-  __attribute__((__nonnull__ (1, 2)));
+extern int protocol_common_open (struct connection *conn,
+                                 uint64_t *exportsize, uint16_t *flags)
+  __attribute__((__nonnull__ (1, 2, 3)));
 
 /* protocol-handshake-oldstyle.c */
 extern int protocol_handshake_oldstyle (struct connection *conn)

server/protocol-handshake-newstyle.c

@@ -203,19 +203,7 @@ conn_recv_full (struct connection *conn, void *buf, size_t len,
 static int
 finish_newstyle_options (struct connection *conn)
 {
-  int64_t r;
-
-  r = backend->get_size (backend, conn);
-  if (r == -1)
-    return -1;
-  if (r < 0) {
-    nbdkit_error (".get_size function returned invalid value "
-                  "(%" PRIi64 ")", r);
-    return -1;
-  }
-  conn->exportsize = (uint64_t) r;
-
-  if (protocol_compute_eflags (conn, &conn->eflags) < 0)
+  if (protocol_common_open (conn, &conn->exportsize, &conn->eflags) == -1)
     return -1;
 
   debug ("newstyle negotiation: flags: export 0x%x", conn->eflags);

server/protocol-handshake-oldstyle.c

@@ -47,7 +47,6 @@ int
 protocol_handshake_oldstyle (struct connection *conn)
 {
   struct old_handshake handshake;
-  int64_t r;
   uint64_t exportsize;
   uint16_t gflags, eflags;
 
@@ -59,21 +58,11 @@ protocol_handshake_oldstyle (struct connection *conn)
     return -1;
   }
 
-  r = backend->get_size (backend, conn);
-  if (r == -1)
+  if (protocol_common_open (conn, &exportsize, &eflags) == -1)
     return -1;
-  if (r < 0) {
-    nbdkit_error (".get_size function returned invalid value "
-                  "(%" PRIi64 ")", r);
-    return -1;
-  }
-  exportsize = (uint64_t) r;
   conn->exportsize = exportsize;
 
   gflags = 0;
-  if (protocol_compute_eflags (conn, &eflags) < 0)
-    return -1;
-
   debug ("oldstyle negotiation: flags: global 0x%x export 0x%x",
          gflags, eflags);
 

server/protocol-handshake.c

@@ -43,15 +43,58 @@
 #include "byte-swapping.h"
 #include "protocol.h"
 
-/* eflags calculation is the same between oldstyle and newstyle
- * protocols.
+int
+protocol_handshake (struct connection *conn)
+{
+  int r;
+
+  lock_request (conn);
+  if (!newstyle)
+    r = protocol_handshake_oldstyle (conn);
+  else
+    r = protocol_handshake_newstyle (conn);
+  unlock_request (conn);
+
+  return r;
+}
+
+/* Common code used by oldstyle and newstyle protocols to:
+ *
+ * - call the backend .open method
+ *
+ * - get the export size
+ *
+ * - compute the eflags (same between oldstyle and newstyle
+ *   protocols)
+ *
+ * The protocols must defer this as late as possible so that
+ * unauthorized clients can't cause unnecessary work in .open by
+ * simply opening a TCP connection.
  */
 int
-protocol_compute_eflags (struct connection *conn, uint16_t *flags)
+protocol_common_open (struct connection *conn,
+                      uint64_t *exportsize, uint16_t *flags)
 {
+  int64_t size;
   uint16_t eflags = NBD_FLAG_HAS_FLAGS;
   int fl;
 
+  if (backend->open (backend, conn, readonly) == -1)
+    return -1;
+
+  /* Prepare (for filters), called just after open. */
+  if (backend->prepare (backend, conn) == -1)
+    return -1;
+
+  size = backend->get_size (backend, conn);
+  if (size == -1)
+    return -1;
+  if (size < 0) {
+    nbdkit_error (".get_size function returned invalid value "
+                  "(%" PRIi64 ")", size);
+    return -1;
+  }
+
   fl = backend->can_write (backend, conn);
   if (fl == -1)
     return -1;
@@ -136,21 +179,7 @@ protocol_compute_eflags (struct connection *conn, uint16_t *flags)
   if (conn->structured_replies)
     eflags |= NBD_FLAG_SEND_DF;
 
+  *exportsize = size;
   *flags = eflags;
   return 0;
 }
-
-int
-protocol_handshake (struct connection *conn)
-{
-  int r;
-
-  lock_request (conn);
-  if (!newstyle)
-    r = protocol_handshake_oldstyle (conn);
-  else
-    r = protocol_handshake_newstyle (conn);
-  unlock_request (conn);
-
-  return r;
-}