Information leak through XEN_DOMCTL_gettscinfo

ISSUE DESCRIPTION

The handler for XEN_DOMCTL_gettscinfo failed to initialize a padding field subsequently copied to guest memory.
(A similar bug existed in XEN_SYSCTL_getdomaininfolist, which is addressed by the patches provided here even though that operation was declared by XSA-77 not to provide security benefits if disaggregated.)

IMPACT

Malicious or buggy stub domain kernels or tool stacks otherwise living outside of Domain0 may be able to read sensitive data relating to the hypervisor or other guests not under the control of that domain.

VULNERABLE SYSTEMS

Xen 4.0.x and later are vulnerable.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
The vulnerability is only exposed to service domains with privilege over another guest. In a usual configuration that means only device model emulators (qemu-dm) when these are running in a separate domain.
In the case of HVM guests whose device model is running in an unrestricted dom0 process, qemu-dm already has the ability to cause problems for the whole system. So in that case the vulnerability is not applicable.
This vulnerability is applicable for an HVM guest with a stub qemu-dm. That is, where the device model runs in a separate domain (in the case of xl, as requested by “device_model_stubdomain_override=1” in the xl domain configuration file). In this case a guest which has already exploited another vulnerability, to gain control of the device model, would be able to exercise the information leak.
However, the security of a system with qemu-dm running in a stub domain is still better than with a qemu-dm running as an unrestricted dom0 process. Therefore users with these configurations should not switch to an unrestricted dom0 qemu-dm.
Finally, in a radically disaggregated system, where the service domain software (probably, the device model domain image in the HVM case) is not always supplied by the host administrator, a malicious service domain administrator can exercise this vulnerability.