Linux Virtual Server/Secure Context procfs shared permissions flaw ================================================================== 2004-07-02, Veit Wahlich
Official location of this document: http://ircnet.de/article.shtml?vsproc Product| -------+ Linux Virtual Server extends the Linux kernel to provide the ability to run several virtual servers on a single host system. In contrast to other virtualization attempts Linux Virtual Server uses a split- userland architechture under a single kernel to optimize sharing of all resources and reduce resource consumption overhead per VM to the absolute minimum. http://www.linux-vserver.org/ Synopsis| --------+ During a security audit on the vproc security scheme a permission- sharing vulnerability was discovered. Vulnerable| ----------+ <= 1.27 (Linux 2.4 stable branch) <= 1.3.9 (Linux 2.4 devel branch) <= 1.9.1 (Linux 2.6 devel branch) Severity| --------+ - local DoS - creation of information leaks See details below. History| -------+ 2004-06-30 vuln discovered 2004-07-02 vendor informed 2004-07-03 first vendor response, confirmation 2004-07-04 official fix available, advisory release Description| -----------+ While auditing and experimenting with VServer procfs and vproc security we discovered a problem sharing permissions on the procfs mounted directories: Within any context users are still able to change permissions on /proc, both access permission and ownership. That is just fine as many people would like to restrict access to /proc to the root user or a group of trusted users. But as changes to a procfs mountpoint do not apply to the mountpoint itself but to procfs in general, these changes affect all contexts (VServers) and even the host system. All tests were done against the stable branch (1.2x) but regarding to Herbert Poetzl, the problem exists on both devel branches (1.3.x, 1.9.x), too. Version 1.28 (stable branch) resolves this problem. Exploitation| ------------+ The vulnerability may be locally exploited in two ways: 1. From within a virtual server a denial of service attack (DoS) may be provoked towards other virtual servers and the host system. By setting permissions that prevent users other than root to read information from procfs (i.e. process information) will disable a wide range of services. 2. On systems where access to procfs is allowed to root only (or to a group of trusted users; i.e. shared hosting environments), an attacker may use access to another virtual server to gain critical information about processes or other data on the primary target virtual server (or the host system). Work-around| -----------+ To work around this problem, procfs may be mounted read-only. On the host-system do: # mount -o remount,ro /proc As this also prevents the host system from changing any values in /proc, this should just be a temporary solution!