Xen Security Advisory 256 - x86 PVH guest without LAPIC may DoS the host
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Xen Security Advisory XSA-256
x86 PVH guest without LAPIC may DoS the host
UPDATES IN VERSION 2
So far, x86 PVH guests can be configured with or without Local APICs.
Configurations with Local APICs are identical to x86 HVM guests, and
will use as much hardware acceleration support as possible.
Configurations without Local APICs try to turn off all hardware
acceleration, and disable all software emulation.
Multiple paths in Xen assume the presence of a Local APIC without
sufficient checks, and can fall over a NULL pointer. On Intel hardware,
the logic to turn off hardware acceleration is incomplete and leaves the
guest with full control of the real Task Priority Register.
A malicious or buggy guest may cause a hypervisor crash, resulting in
a Denial of Service (DoS) affecting the entire host.
Xen version 4.8 and onwards are vulnerable.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
Only x86 PVH guests can exploit the vulnerability. x86 PV and HVM
guests cannot exploit the vulnerability.
Running only PV or HVM guests avoids the vulnerability.
Running all PVH guests with "apic=1" in the guest configuration file
(or equivalent thereof) also avoids the vulnerability.
This issue was discovered by Ian Jackson of Citrix.
Applying the appropriate attached patch resolves this issue.
DEPLOYMENT DURING EMBARGO
Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).
Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable. This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)