Missing p2m error checking in PoD code

ISSUE DESCRIPTION

Certain actions require modification of entries in a guest’s P2M (Physical-to-Machine) table. When large pages are in use for this table, such an operation may incur a memory allocation (to replace a large mapping with individual smaller ones). If this allocation fails, the p2m_set_entry() function will return an error.
Unfortunately, several places in the populate-on-demand code don’t check the return value of p2m_set_entry() to see if it succeeded.
In some cases, the operation was meant to remove an entry from the p2m table. If this removal fails, a malicious guest may engineer that the page be returned to the Xen free list, making it available to be allocated to another domain, while it retains a writable mapping to the page.
In other cases, the operation was meant to remove special populate-on-demand entries; if this removal fails, the internal accounting becomes inconsistent and may eventually hit a BUG().
The allocation involved comes from a separate pool of memory created when the domain is created; under normal operating conditions it never fails, but a malicious guest may be able to engineer situations where this pool is exhausted.

IMPACT

An unprivileged guest can retain a writable mapping of freed memory. Depending on how this page is used, it could result in either an information leak, or full privilege escalation.
Alternatively, an unprivileged guest can cause Xen to hit a BUG(), causing a clean crash - ie, host-wide denial-of-service (DoS).

VULNERABLE SYSTEMS

All systems from Xen 3.4 are vulnerable.
Only x86 systems are vulnerable. ARM is not vulnerable.
x86 PV VMs cannot leverage the vulnerability.
Only systems with 2MiB or 1GiB HAP pages enabled are vulnerable.
The vulnerability is largely restricted to HVM guests which have been constructed in Populate-on-Demand mode (i.e. with memory < maxmem):
x86 HVM domains without PoD (i.e. started with memory == maxmem, or without mentioning “maxmem” in the guest config file) also cannot leverage the vulnerability, in recent enough Xen versions: 4.8.x and later: all versions safe if PoD not configured 4.7.x: 4.7.1 and later safe if PoD not configured 4.6.x: 4.6.4 and later safe if PoD not configured 4.5.x: 4.5.4 and later safe if PoD not configured 4.4.x and earlier: all versions vulnerable even if PoD not configured
The commit required to prevent this vulnerability when PoD not configured is 2a99aa99fc84a45f505f84802af56b006d14c52e xen/physmap: Do not permit a guest to populate PoD pages for itself and the corresponding backports.