Extend pt_chown to drop privileges.

If libcap is available, use it to drop privileges in pt_chown before
starting the work to change the permissions and ownership of the
slave device.

ChangeLog

@@ -1,3 +1,7 @@
+2009-06-16  Ulrich Drepper  <drepper@redhat.com>
+
+	* login/Makefile: If necessary link pt_chown with -lcap.
+
 2009-06-16  Jakub Jelinek  <jakub@redhat.com>
 
 	* sysdeps/x86_64/memchr.S (memchr): Use unsigned instead of signed

login/Makefile

@@ -1,4 +1,4 @@
-# Copyright (C) 1996-1998,2000-2002,2003,2007 Free Software Foundation, Inc.
+# Copyright (C) 1996-1998,2000-2003,2007, 2009 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 
 # The GNU C Library is free software; you can redistribute it and/or
@@ -54,6 +54,11 @@ otherlibs += $(nssobjdir)/libnss_files.a $(resolvobjdir)/libnss_dns.a \
 	     $(resolvobjdir)/libresolv.a $(common-objpfx)libc.a
 endif
 
+ifeq (yes,$(have-libcap))
+libcap = -lcap
+endif
+LDLIBS-pt_chown = $(libcap)
+
 # pt_chown needs to be setuid root.
 $(inst_libexecdir)/pt_chown: $(objpfx)pt_chown $(+force)
 	$(make-target-directory)

login/programs/pt_chown.c

@@ -29,6 +29,10 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#ifdef HAVE_LIBCAP
+# include <sys/capability.h>
+# include <sys/prctl.h>
+#endif
 
 #include "pty-private.h"
 
@@ -99,7 +103,7 @@ static int
 do_pt_chown (void)
 {
   char *pty;
-  struct stat st;
+  struct stat64 st;
   struct group *p;
   gid_t gid;
 
@@ -110,7 +114,7 @@ do_pt_chown (void)
 
   /* Check that the returned slave pseudo terminal is a
      character device.  */
-  if (stat (pty, &st) < 0 || !S_ISCHR(st.st_mode))
+  if (stat64 (pty, &st) < 0 || !S_ISCHR (st.st_mode))
     return FAIL_EINVAL;
 
   /* Get the group ID of the special `tty' group.  */
@@ -136,16 +140,43 @@ int
 main (int argc, char *argv[])
 {
   uid_t euid = geteuid ();
+  uid_t uid = getuid ();
   int remaining;
 
-  /* Normal invocation of this program is with no arguments and
-     with privileges.
-     FIXME: Should use capable (CAP_CHOWN|CAP_FOWNER).  */
   if (argc == 1 && euid == 0)
-    return do_pt_chown ();
+    {
+#ifdef HAVE_LIBCAP
+  /* Drop privileges.  */
+      if (uid != euid)
+	{
+	  static const cap_value_t cap_list[] =
+	    { CAP_CHOWN, CAP_FOWNER	};
+# define ncap_list (sizeof (cap_list) / sizeof (cap_list[0]))
+	  cap_t caps = cap_init ();
+	  if (caps == NULL)
+	    error (FAIL_ENOMEM, errno,
+		   _("Failed to initialize drop of capabilities"));
+
+	  /* There is no reason why these should not work.  */
+	  cap_set_flag (caps, CAP_PERMITTED, ncap_list, cap_list, CAP_SET);
+	  cap_set_flag (caps, CAP_EFFECTIVE, ncap_list, cap_list, CAP_SET);
+
+	  int res = cap_set_proc (caps);
+
+	  cap_free (caps);
+
+	  if (__builtin_expect (res != 0, 0))
+	    error (FAIL_EXEC, errno, _("cap_set_proc failed"));
+	}
+#endif
+
+      /* Normal invocation of this program is with no arguments and
+	 with privileges.  */
+      return do_pt_chown ();
+    }
 
   /* We aren't going to be using privileges, so drop them right now. */
-  setuid (getuid ());
+  setuid (uid);
 
   /* Set locale via LC_ALL.  */
   setlocale (LC_ALL, "");

sysdeps/generic/pty-private.h

@@ -1,5 +1,5 @@
 /* Internal defenitions and declarations for pseudo terminal functions.
-   Copyright (C) 1998, 1999 Free Software Foundation, Inc.
+   Copyright (C) 1998, 1999, 2009 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Zack Weinberg <zack@rabi.phys.columbia.edu>, 1998.
 
@@ -39,7 +39,8 @@ enum  /* failure modes */
   FAIL_EBADF = 1,
   FAIL_EINVAL,
   FAIL_EACCES,
-  FAIL_EXEC
+  FAIL_EXEC,
+  FAIL_ENOMEM
 };
 
 #endif /* pty-private.h  */

sysdeps/unix/grantpt.c

@@ -202,7 +202,7 @@ grantpt (int fd)
       if (!WIFEXITED (w))
 	__set_errno (ENOEXEC);
       else
-	switch (WEXITSTATUS(w))
+	switch (WEXITSTATUS (w))
 	  {
 	  case 0:
 	    retval = 0;
@@ -219,6 +219,9 @@ grantpt (int fd)
 	  case FAIL_EXEC:
 	    __set_errno (ENOEXEC);
 	    break;
+	  case FAIL_ENOMEM:
+	    __set_errno (ENOMEM);
+	    break;
 
 	  default:
 	    assert(! "getpt: internal error: invalid exit code from pt_chown");