GNTTABOP_swap_grant_ref operation misbehavior

ISSUE DESCRIPTION

With the introduction of version 2 grant table operations, a version check became necessary for most grant table related hypercalls. The GNTTABOP_swap_grant_ref call was lacking such a check. As a result, the subsequent code behaved as if version 2 was in use, when a guest issued this hypercall without a prior GNTTABOP_setup_table or GNTTABOP_set_version.
The effect is a possible NULL pointer dereferences. However, this cannot be exploited to elevate privileges of the attacking domain, as the maximum memory address that can be wrongly accessed this way is bounded to far below the start of hypervisor memory.

IMPACT

Malicious or buggy guest domain kernels can mount a denial of service attack which, if successful, can affect the whole system.

VULNERABLE SYSTEMS

Xen versions from 4.2 onwards are vulnerable.