screen/backport-CVE-2025-46805.patch

From 161f85b98b7e1d5e4893aeed20f4cdb5e3dfaaa4 Mon Sep 17 00:00:00 2001
From: Matthias Gerstner <matthias.gerstner@suse.de>
Date: Mon, 12 May 2025 15:38:19 +0200
Subject: [PATCH 3/3] fix CVE-2025-46805: socket.c - don't send signals with
 root privileges

The CheckPid() function was introduced to address CVE-2023-24626, to
prevent sending SIGCONT and SIGHUP to arbitrary PIDs in the system. This
fix still suffers from a TOCTOU race condition. The client can replace
itself by a privileged process, or try to cycle PIDs until a privileged
process receives the original PID.

To prevent this, always send signals using the real privileges. Keep
CheckPid() for error diagnostics. If sending the actual signal fails
later on then there will be no more error reporting.

It seems the original bugfix already introduced a regression when
attaching to another's user session that is not owned by root. In this
case the target sessions runs with real uid X, while for sending a
signal to the `pid` provided by the client real uid Y (or root
privileges) are required.

This is hard to properly fix without this regression. On Linux pidfds
could be used to allow safely sending signals to other PIDs as root
without involving race conditions. In this case the client PID should
also be obtained via the UNIX domain socket's SO_PEERCRED option,
though.
---
 socket.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/socket.c b/socket.c
index 6c3502f..d6621fa 100644
--- a/socket.c
+++ b/socket.c
@@ -831,6 +831,11 @@ int pid;
   return UserStatus();
 }
 
+static void KillUnpriv(pid_t pid, int sig) {
+    UserContext();
+    UserReturn(kill(pid, sig));
+}
+
 #ifdef hpux
 /*
  * From: "F. K. Bruner" <napalm@ugcs.caltech.edu>
@@ -916,14 +921,14 @@ struct win *wi;
             {
 	      Msg(errno, "Could not perform necessary sanity checks on pts device.");
 	      close(i);
-	      Kill(pid, SIG_BYE);
+	      KillUnpriv(pid, SIG_BYE);
 	      return -1;
             }
           if (strcmp(ttyname_in_ns, m->m_tty))
             {
 	      Msg(errno, "Attach: passed fd does not match tty: %s - %s!", ttyname_in_ns, m->m_tty[0] != '\0' ? m->m_tty : "(null)");
 	      close(i);
-	      Kill(pid, SIG_BYE);
+	      KillUnpriv(pid, SIG_BYE);
 	      return -1;
 	    }
 	  /* m->m_tty so far contains the actual name of the pts device in the
@@ -940,19 +945,19 @@ struct win *wi;
 	{
 	  Msg(errno, "Attach: passed fd does not match tty: %s - %s!", m->m_tty, myttyname ? myttyname : "NULL");
 	  close(i);
-	  Kill(pid, SIG_BYE);
+	  KillUnpriv(pid, SIG_BYE);
 	  return -1;
 	}
     }
   else if ((i = secopen(m->m_tty, O_RDWR | O_NONBLOCK, 0)) < 0)
     {
       Msg(errno, "Attach: Could not open %s!", m->m_tty);
-      Kill(pid, SIG_BYE);
+      KillUnpriv(pid, SIG_BYE);
       return -1;
     }
 #ifdef MULTIUSER
   if (attach)
-    Kill(pid, SIGCONT);
+    KillUnpriv(pid, SIGCONT);
 #endif
 
 #if defined(ultrix) || defined(pyr) || defined(NeXT)
@@ -965,7 +970,7 @@ struct win *wi;
 	{
 	  write(i, "Attaching from inside of screen?\n", 33);
 	  close(i);
-	  Kill(pid, SIG_BYE);
+	  KillUnpriv(pid, SIG_BYE);
 	  Msg(0, "Attach msg ignored: coming from inside.");
 	  return -1;
 	}
@@ -976,7 +981,7 @@ struct win *wi;
 	  {
 	      write(i, "Access to session denied.\n", 26);
 	      close(i);
-	      Kill(pid, SIG_BYE);
+	      KillUnpriv(pid, SIG_BYE);
 	      Msg(0, "Attach: access denied for user %s.", user);
 	      return -1;
 	  }
@@ -1294,7 +1299,7 @@ ReceiveMsg()
 	    Msg(0, "Query attempt with bad pid(%d)!", m.m.command.apid);
 	  }
 	  else {
-            Kill(m.m.command.apid,
+            KillUnpriv(m.m.command.apid,
                (queryflag >= 0)
                    ? SIGCONT
                    : SIG_BYE); /* Send SIG_BYE if an error happened */