kern/70298: manipulation of ufs system flags in jail(8) is permitted for super user

[Marcin Koziej]

>Number:         70298
>Category:       kern
>Synopsis:       manipulation of ufs system flags in jail(8) is permitted for super user
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Wed Aug 11 12:20:30 GMT 2004
>Closed-Date:
>Last-Modified:
>Originator:     Marcin Koziej
>Release:        FreeBSD 5.2.1-RELEASE-p9 i386
>Organization:
DESK.pl
>Environment:
System: FreeBSD mistress 5.2.1-RELEASE-p9 FreeBSD 5.2.1-RELEASE-p9 #0: Fri Jul 30 02:52:09 CEST 2004 creep at mistress:/usr/src/sys/i386/compile/MONSUN i386

>Description:
	Uid 0 user can modify system flags in jail, which shouldn't
	happen according to sys/ufs/ufs/ufs_vnops.c:498
        /*
         * Unprivileged processes and privileged processes in
         * jail() are not permitted to unset system flags, or
         * modify flags if any system flags are set.
         * Privileged non-jail processes may not modify system flags
         * if securelevel > 0 and any existing system flags are set.
         */

	but after that there is:

	if (!suser_cred(cred, PRISON_ROOT)) {
		(code for unprivileged)
	} else {
		(code for superuser)
	}
	
	PRISON_ROOT flag makes uid 0 in jail(8) privileged and should be
	replaced with 0.

>How-To-Repeat:

root:~:# uname -a; id; ps awux |grep $$
FreeBSD mistress 5.2.1-RELEASE-p9 FreeBSD 5.2.1-RELEASE-p9 #0: Fri Jul 30 02:52:09 CEST 2004     creep at mistress:/usr/src/sys/i386/compile/MONSUN  i386
uid=0(root) gid=0(wheel) groups=0(wheel), 2(kmem), 3(sys), 4(tty), 5(operator)
root    633  0,0  1,3  2244 1608  p7  Ss   13:23     0:00,05 /usr/local/bin/zsh -i
root    637  0,0  0,2   424  212  p7  DL+  13:23     0:00,01 grep 633
root:~:# touch test; ls -lo test 
-rw-r--r--  1 root  wheel  - 0 11 Sie 13:24 test
root:~:# chflags schg test; ls -lo test
-rw-r--r--  1 root  wheel  schg 0 11 Sie 13:24 test
root:~:# jail / test 127.0.0.1 $SHELL
root:/:# ps awux |grep $$
root   642  0,0  1,3  2244 1608  p7  SJ   13:24     0:00,06 /usr/local/bin/zsh
root   644  0,0  0,6  1460  764  p7  DL+J 13:24     0:00,01 grep 642
root:/:# cd
root:~:# ls -lo test
-rw-r--r--  1 root  wheel  schg 0 11 Sie 13:24 test
root:~:# chflags noschg test; ls -lo test
-rw-r--r--  1 root  wheel  - 0 11 Sie 13:24 test
root:~:# exit

>Fix:

To fix it PRISON_ROOT flag needs to be replaced with 0.

----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----
--- sys/ufs/ufs/ufs_vnops.c.orig	Tue Sep 23 15:52:43 2003
+++ sys/ufs/ufs/ufs_vnops.c	Tue Sep 23 15:52:48 2003
@@ -502,7 +502,7 @@
 		 * Privileged non-jail processes may not modify system flags
 		 * if securelevel > 0 and any existing system flags are set.
 		 */
-		if (!suser_cred(cred, PRISON_ROOT)) {
+		if (!suser_cred(cred, 0)) {
 			if (ip->i_flags
 			    & (SF_NOUNLINK | SF_IMMUTABLE | SF_APPEND)) {
 				error = securelevel_gt(cred, 0);
----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----

Also, an explicit description of what PRISON_ROOT flag do would 
prevent this kind of errors in the future.

----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----
--- sys/kern/kern_prot.c.orig   Wed Aug 11 13:56:29 2004
+++ sys/kern/kern_prot.c        Wed Aug 11 13:58:54 2004
@@ -1227,7 +1227,8 @@
 /*
  * Test whether the specified credentials imply "super-user" privilege.
  * Return 0 or EPERM.  The flag argument is currently used only to
- * specify jail interaction.
+ * specify jail interaction. PRISON_ROOT bit set in flag argument provides
+ * uid 0 in jail with "super-user" privilege.
  */
 int
 suser_cred(struct ucred *cred, int flag)
----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----

suser_cred with PRISON_ROOT is called in various places in the kernel,
maybe they should be revised.


Big thanks to all FreeBSD developers for providing a great OS.

-- 
m.

>Release-Note:
>Audit-Trail:
>Unformatted: