issues with partially successful P2M updates on x86

ISSUE DESCRIPTION

x86 HVM and PVH guests may be started in populate-on-demand (PoD) mode, to provide a way for them to later easily have more memory assigned.
Guests are permitted to control certain P2M aspects of individual pages via hypercalls. These hypercalls may act on ranges of pages specified via page orders (resulting in a power-of-2 number of pages). In some cases the hypervisor carries out the requests by splitting them into smaller chunks. Error handling in certain PoD cases has been insufficient in that in particular partial success of some operations was not properly accounted for.
There are two code paths affected - page removal (CVE-2021-28705) and insertion of new pages (CVE-2021-28709). (We provide one patch which combines the fix to both issues.)

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 3.4 onwards are affected. Xen versions 3.3 and older are believed to not be affected.
Only x86 HVM and PVH guests started in populate-on-demand mode are believed to be able to leverage the vulnerability. Populate-on-demand mode is activated when the guest’s xl configuration file specifies a “maxmem” value which is larger than the “memory” value.