Re: Logging-in using ssh elevates the user privilege.

[Takashi Yano <takashi.yano@nifty.ne.jp>]

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Hi Corinna,

Thanks for your advice.

On Fri, 8 Mar 2019 15:11:18 +0100 Corinna Vinschen wrote:
> > In Linux, connect() in the client returns befor the
> > server calls accept(). However, in cygwin, connect()
> > does not return until the server calls accept().
> 
> This is a result of the handshake to exchange credentials for
> getpeereid().  To workaround this issue, try building screen
> with a tweak.  Server and as client should call
> 
>   setsockopt (sock, SOL_SOCKET, SO_PEERCRED, NULL, 0);
> 
> before calling accept or connect.

Following your advice, I tried the patch attached and
confirmed the problem regarding -Q option is solved.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

[-- Attachment #2: screen-peercred.patch --]
[-- Type: application/octet-stream, Size: 1467 bytes --]

--- origsrc/screen-4.6.2/socket.c	2017-10-23 20:32:41.000000000 +0900
+++ src/screen-4.6.2/socket.c	2019-03-08 23:31:11.373592400 +0900
@@ -537,6 +537,9 @@
   xseteuid(real_uid);
   xsetegid(real_gid);
 # endif
+#ifdef __CYGWIN__
+  setsockopt(s, SOL_SOCKET, SO_PEERCRED, NULL, 0);
+#endif
   if (connect(s, (struct sockaddr *) &a, strlen(SockPath) + 2) != -1)
     {
       debug("oooooh! socket already is alive!\n");
@@ -628,6 +631,9 @@
       return -1;
     }
 #endif
+#ifdef __CYGWIN__
+  setsockopt(s, SOL_SOCKET, SO_PEERCRED, NULL, 0);
+#endif
   if (connect(s, (struct sockaddr *)&a, strlen(SockPath) + 2) == -1)
     {
       if (err)
@@ -1058,6 +1064,9 @@
     {
       len = sizeof(a);
       debug("Ha, there was someone knocking on my socket??\n");
+#ifdef __CYGWIN__
+      setsockopt(ns, SOL_SOCKET, SO_PEERCRED, NULL, 0);
+#endif
       if ((ns = accept(ns, (struct sockaddr *)&a, (void *)&len)) < 0)
         {
           Msg(errno, "accept");
@@ -1308,6 +1317,9 @@
     }
   else
     {
+#ifdef __CYGWIN__
+      setsockopt(s, SOL_SOCKET, SO_PEERCRED, NULL, 0);
+#endif
       len = sizeof(a);
       s = accept(s, (struct sockaddr *)&a, (void *)&len);
       if (s < 0)
@@ -1343,6 +1355,9 @@
   if (stat(sap->sun_path, &st))
     return -1;
   chmod(sap->sun_path, 0);
+#ifdef __CYGWIN__
+  setsockopt(s, SOL_SOCKET, SO_PEERCRED, NULL, 0);
+#endif
   x = connect(s, (struct sockaddr *) sap, len);
   chmod(sap->sun_path, st.st_mode);
   return x;

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