Page table manipulation operations for PV guests can take significant amounts of time, as they require all present branches to have their type (and thus contents) verified. While the most frequently used operations had been made preemptible in the past, some code paths involving potentially deep page table traversal were still trying to do their entire work in a single step.
Malicious or buggy PV guest kernels could potentially mount a denial of service attack affecting the whole system.
Red Hat would like to thank the Xen project for reporting this issue.
This issue did affect the versions of the kernel-xen package as shipped with Red Hat Enterprise Linux 5.
This issue did not affect Red Hat Enterprise Linux 6 and Red Hat Enterprise MRG 2.
The risks associated with fixing this bug are greater than its security impact. This issue is not currently planned to be addressed in future kernel-xen updates for Red Hat Enterprise Linux 5. Using fully virtualized (HVM) guests, or PV guests with trusted kernel/administrator avoids this issue.
Now public via http://seclists.org/oss-sec/2013/q2/259.
Created xen tracking bugs for this issue
Affects: fedora-all [bug 958918]