possible memory corruption via failsafe callback

ISSUE DESCRIPTION

Under certain special conditions Xen reports an exception resulting from returning to guest mode not via ordinary exception entry points, but via a so call failsafe callback. This callback, unlike exception handlers, takes 4 extra arguments on the stack (the saved data selectors DS, ES, FS, and GS). Prior to placing exception or failsafe callback frames on the guest kernel stack, Xen checks the linear address range to not overlap with hypervisor space. The range spanned by that check was mistakenly not covering these extra 4 slots.

IMPACT

A malicious or buggy 64-bit PV guest may be able to modify part of a physical memory page not belonging to it, potentially allowing for all of privilege escalation, host or other guest crashes, and information leaks.

VULNERABLE SYSTEMS

64-bit Xen versions 4.6 and earlier are vulnerable. Xen versions 4.7 and later are not vulnerable.
Only x86 systems are affected. ARM systems are not vulnerable.
Only x86 systems with physical memory extending to a configuration dependent boundary (5Tb or 3.5Tb) may be affected. Whether they are actually affected depends on actual physical memory layout.
The vulnerability is only exposed to 64-bit PV guests. HVM guests and 32-bit PV guests can’t exploit the vulnerability.