Executive brief
Xen is a hypervisor used to run multiple virtual machines on a single physical server. A vulnerability in how it handles certain guest operations allows a malicious administrator of a virtual machine to crash or freeze the entire physical host. This results in a total service outage for all other virtual machines running on that same hardware.
Technical details
A vulnerability exists in Xen's x86 instruction emulator when handling Paravirtualized (PV) guest operations, such as page table writes. A malicious guest kernel administrator can asynchronously modify the instruction stream during emulation to manipulate the EFLAGS.IF bit. This allows the guest to mask interrupts in a way that the hypervisor does not expect, leading to a host-level hang or crash (Denial of Service). The issue specifically affects x86 PV guests; HVM and ARM guests are not impacted. Patches have been released by the Xen Project (XSA-202) and various downstream vendors like Citrix and Debian.
Affected products
- Xen Project Xen through 4.8.x
- Citrix XenServer 6.0.2, 6.2.0, 6.5, 7.0
Timeline
- 2016-12-21: advisory: Xen Project released XSA-202
- 2016-12-21: patched: Patches released for Xen unstable and stable branches
- 2017-01-26: disclosed: NVD publication date