scd: Detect external interference when PCSC_SHARED.

* scd/app-common.h (check_aid): New method.
* scd/app-openpgp.c (do_check_aid): New.
* scd/app-piv.c (do_check_aid): New.
* scd/app.c (check_external_interference): New.
(maybe_switch_app): Check interference to determine switching is
needed.

--

GnuPG-bug-id: 5484
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

scd/app-common.h

@@ -194,6 +194,8 @@ struct app_ctx_s {
                       void *pincb_arg);
     gpg_error_t (*with_keygrip) (app_t app, ctrl_t ctrl, int action,
                                  const char *keygrip_str, int capability);
+    gpg_error_t (*check_aid) (app_t app, ctrl_t ctrl,
+                              const unsigned char *aid, size_t aidlen);
   } fnc;
 };
 

scd/app-openpgp.c

@@ -6134,6 +6134,18 @@ do_reselect (app_t app, ctrl_t ctrl)
 }
 
 
+/* Check if AID is the correct one.  */
+static gpg_error_t
+do_check_aid (app_t app, ctrl_t ctrl, const unsigned char *aid, size_t aidlen)
+{
+  if (aidlen >= sizeof openpgp_aid
+      && memcmp (aid, openpgp_aid, sizeof openpgp_aid) == 0)
+    return 0;
+
+  return gpg_error (GPG_ERR_WRONG_CARD);
+}
+
+
 /* Select the OpenPGP application on the card in SLOT.  This function
    must be used before any other OpenPGP application functions. */
 gpg_error_t
@@ -6322,6 +6334,7 @@ app_select_openpgp (app_t app)
       app->fnc.change_pin = do_change_pin;
       app->fnc.check_pin = do_check_pin;
       app->fnc.with_keygrip = do_with_keygrip;
+      app->fnc.check_aid = do_check_aid;
    }
 
 leave:

scd/app-piv.c

@@ -3609,6 +3609,18 @@ do_reselect (app_t app, ctrl_t ctrl)
 }
 
 
+/* Check if AID is the correct one.  */
+static gpg_error_t
+do_check_aid (app_t app, ctrl_t ctrl, const unsigned char *aid, size_t aidlen)
+{
+  if (aidlen >= sizeof piv_aid
+      && memcmp (aid, piv_aid, sizeof piv_aid) == 0)
+    return 0;
+
+  return gpg_error (GPG_ERR_WRONG_CARD);
+}
+
+
 /* Select the PIV application on the card in SLOT.  This function must
  * be used before any other PIV application functions. */
 gpg_error_t
@@ -3713,6 +3725,7 @@ app_select_piv (app_t app)
   app->fnc.change_pin = do_change_chv;
   app->fnc.check_pin = do_check_chv;
   app->fnc.with_keygrip = do_with_keygrip;
+  app->fnc.check_aid = do_check_aid;
 
 
 leave:

scd/app.c

@@ -1435,6 +1435,46 @@ run_reselect (ctrl_t ctrl, card_t c, app_t a, app_t a_prev)
 }
 
 
+/*
+ * Check external interference before each use of the application on
+ * card.  Returns -1 when detecting some external interference.
+ * Returns 0 if not.
+ *
+ * Note: This kind of detection can't be perfect.  At most, it may be
+ * possibly useful kludge, in some limited situations.
+ */
+static int
+check_external_interference (app_t app, ctrl_t ctrl)
+{
+  /*
+   * Only when a user is using Yubikey with pcsc-shared configuration,
+   * we need this detection.  Otherwise, the card/token is under full
+   * control of scdaemon, there's no problem at all.
+   */
+  if (!opt.pcsc_shared || app->card->cardtype != CARDTYPE_YUBIKEY)
+    return 0;
+
+  if (app->fnc.check_aid)
+    {
+      unsigned char *aid;
+      size_t aidlen;
+      gpg_error_t err;
+      int slot = app_get_slot (app);
+
+      err = iso7816_get_data (slot, 0, 0x004F, &aid, &aidlen);
+      if (err)
+        return -1;
+
+      err = app->fnc.check_aid (app, ctrl, aid, aidlen);
+      xfree (aid);
+      if (err)
+        return -1;
+    }
+
+  return 0;
+}
+
+
 /* Check that the card has been initialized and whether we need to
  * switch to another application on the same card.  Switching means
  * that the new active app will be moved to the head of the list at
@@ -1490,7 +1530,8 @@ maybe_switch_app (ctrl_t ctrl, card_t card, const char *keyref)
             if (app->apptype == apptype)
               break;
           if (!app_prev && ctrl->current_apptype == card->app->apptype)
-            return 0;  /* Already the first app - no need to switch.  */
+            if (check_external_interference (app, ctrl) == 0)
+              return 0;  /* Already the first app - no need to switch.  */
         }
       else if (strlen (keyref) == 40)
         {
@@ -1503,7 +1544,8 @@ maybe_switch_app (ctrl_t ctrl, card_t card, const char *keyref)
                                            KEYGRIP_ACTION_LOOKUP, keyref, 0))
               break;
           if (!app_prev && ctrl->current_apptype == card->app->apptype)
-            return 0;   /* Already the first app - no need to switch.  */
+            if (check_external_interference (app, ctrl) == 0)
+              return 0;   /* Already the first app - no need to switch.  */
         }
     }