Description of problem: Every paravirt guest (and some fullvirt guests) have a TTY path associated with them for the text console access to the guest domain. The TTY path is allocated at time of VM creation, and is written into xenstored. xm console reads the TTY path out of xenstored and opens it to provide admin access to the text console. The problem is that the TTY path is written into an area of xenstore which is writtable by the guest. So a malicious guest can re-write the TTY path, tricking the host admin into accessing a different TTY than they should. eg, if you have a guest called 'demo', with domain ID 5, inside the guest you could do # yum install xen # xenstore-write /local/domain/5/console/tty /i/am/the/evil/guest Then when the host admin tries to connect to the console later # xm console rhel5pv xenconsole: Could not open tty `/i/am/the/evil/guest': No such file or directory Not sure yet if this could cause xm console to actually corrupt/overwrite important files, or if its just a inconvenience. There is a tonne of other info written & read to/from this untrustable area, and some of it *is* serious For fullvirt guests, the PID of the QEMU device model is written into the device model at /local/domain/$DOMID/image/device-model-pid If a malicious guest did xenstore-write /local/domain/26/image/device-model-pid 1 It is possible that in some circumstances, when a host admin later tries to kill the guest, it would in fact kill 'init' process in the host. Version-Release number of selected component (if applicable): xen-3.0.3-64.el5 How reproducible: Always Steps to Reproduce: 1. Inside a guest #yum install xen # xenstore-write /local/domain/GUEST-DOMID/console/tty /i/am/the/evil/guest 2. On the host xm console GUEST-NAME Also various other checks Actual results: xenconsole: Could not open tty `/i/am/the/evil/guest': No such file or directory Expected results: xm console still works, and does not read data from untrusted areas. Additional info:
This flaw is publically known, since a developer posted a patch to xen-devel without saying that it was security sensitive :-( Lead of thread: http://lists.xensource.com/archives/html/xen-devel/2008-09/msg00992.html Place where it was pointed out to be security problem http://lists.xensource.com/archives/html/xen-devel/2008-09/msg00994.html
There is now a fix available from upstream repository that addresses this in 2 parts - Makes most of the xenstore entries under /local/domain/$DOMID read-only to the guest. only 'devices', 'error' and 'control' are now writable by the guest - Moves some of the stuff previously under the '/local/domain/$DOMID/devices' section to /vm/$UUID/devices instead http://xenbits.xensource.com/staging/xen-3.3-testing.hg?rev/e0e17216ba70 This will need to be backported to the Xen in RHEL-5
I'm moving this bug to the Security Response product for proper tracking as a security flaw.
I'm giving this a severity of moderate. Once we have a CVE id, we can fix it in RHEL5 with an async update.
Turns out the upstream patch is broken http://lists.xensource.com/archives/html/xen-devel/2008-12/msg00842.html
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2008-5716 has been reported due comment c#8.
This was released a long time ago, so closing this tracking bug. Chris Lalancette