plugins/pam: Check the user didn't change during PAM transaction

PAM modules can change the user during their execution, in such case,
sudo would still use the user that has been provided giving potentially
access to another user with the credentials of another one.

So prevent this to happen, by ensuring that the final PAM user is
matching the one which started the transaction

plugins/sudoers/auth/pam.c

@@ -330,6 +330,19 @@ sudo_pam_verify(const struct sudoers_context *ctx, struct passwd *pw,
 		debug_return_int(AUTH_FAILURE);
 	}
 
+    if (*pam_status == PAM_SUCCESS) {
+	const char *pam_user = NULL;
+
+	*pam_status = pam_get_item(pamh, PAM_USER, (const void **) &pam_user);
+	if (*pam_status == PAM_SUCCESS &&
+	    (pam_user == NULL || strcmp(pam_user, pw->pw_name) != 0)) {
+	    sudo_debug_printf(SUDO_DEBUG_WARN|SUDO_DEBUG_LINENO,
+			      "unable to authenticate '%s' as user '%s'",
+			      pw->pw_name, pam_user);
+	    debug_return_int(AUTH_FAILURE);
+	}
+    }
+
     if (getpass_error) {
 	/* error or ^C from tgetpass() or running non-interactive */
 	debug_return_int(noninteractive ? AUTH_NONINTERACTIVE : AUTH_INTR);