The emulation of the outs instruction for 64-bit PV guests uses an uninitialized variable as the segment base for the source data if an FS: or GS: segment override is used, and if the segment descriptor the respective non-null selector in the corresponding selector register points to cannot be read by the emulation code (this is possible if the segment register was loaded before a more recent GDT or LDT update, i.e. the segment register contains stale data). A malicious guest might be able to get hold of contents of the hypervisor stack, through the fault address passed to the page fault handler if the outs raises such a fault (which is mostly under guest control). Other methods for indirectly deducing information also exist.
A malicious 64-bit PV guest might conceivably gain access to sensitive data relating to other guests.
Xen 3.1.x and later are vulnerable. Only 64-bit PV guests can take advantage of this vulnerability.