vulnerability in the iret hypercall handler

ISSUE DESCRIPTION

A buggy loop in Xen’s compat_iret() function iterates the wrong way around a 32-bit index. Any 32-bit PV guest kernel can trigger this vulnerability by attempting a hypercall_iret with EFLAGS.VM set.
Given the use of __get/put_user(), and that the virtual addresses in question are contained within the lower canonical half, the guest cannot clobber any hypervisor data. Instead, Xen will take up to 2^33 pagefaults, in sequence, effectively hanging the host.

IMPACT

Malicious guest administrators can cause a denial of service affecting the whole system.

VULNERABLE SYSTEMS

Only 64-bit x86 (ARCH=x86_64) builds of Xen are vulnerable. 32-bit builds (ARCH=x86_32) (necessarily of Xen 4.2 or earlier), are not affected.
Xen versions 3.1 or later are vulnerable.
ARM systems are not vulnerable.
Only 32-bit PV guests can exploit the vulnerability.