inadequate grant-v2 status frames array bounds check

ISSUE DESCRIPTION

The v2 grant table interface separates grant attributes from grant status. That is, when operating in this mode, a guest has two tables. As a result, guests also need to be able to retrieve the addresses that the new status tracking table can be accessed through.
For 32-bit guests on x86, translation of requests has to occur because the interface structure layouts commonly differ between 32- and 64-bit.
The translation of the request to obtain the frame numbers of the grant status table involves translating the resulting array of frame numbers. Since the space used to carry out the translation is limited, the translation layer tells the core function the capacity of the array within translation space. Unfortunately the core function then only enforces array bounds to be below 8 times the specified value, and would write past the available space if enough frame numbers needed storing.

IMPACT

Malicious or buggy guest kernels may be able to mount a Denial of Service (DoS) attack affecting the entire system. Privilege escalation and information leaks cannot be ruled out.

VULNERABLE SYSTEMS

All Xen versions from 4.10 onwards are affected. Xen versions 4.9 and older are not affected.
Only 32-bit x86 guests permitted to use grant table version 2 interfaces can leverage this vulnerability. 64-bit x86 guests cannot leverage this vulnerability, but note that HVM and PVH guests are free to alter their bitness as they see fit. On Arm, grant table v2 use is explicitly unsupported.
Only guests permitted to have 8177 or more grant table frames can leverage this vulnerability.