Failure to recognize errors being returned from low level functions in
Populate on Demand (PoD) code may result in higher level code entering
an infinite loop.
A malicious HVM guest can cause one pcpu to permanently hang. This
normally cascades into the whole system freezing, resulting in a a
host Denial of Service (DoS).
Xen versions from 3.4.x onwards are affected.
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.
Running only PV guests will avoid this issue.
Running HVM guests only in non-PoD mode (maxmem == memory) will also
avoid this issue. NOTE: In older releases of Xen, an HVM guest can
create PoD entries itself; so this mitigation will not be effective.
Specifying "hap_1gb=0 hap_2mb=0" on the hypervisor command line will
avoid the vulnerability.
Alternatively, running all x86 HVM guests in shadow mode will also
avoid this vulnerability. (For example, by specifying "hap=0" in the
xl domain configuration file.)
Name: the Xen project
Upstream: Julien Grall (Linaro)
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1518214]