CVE-2020-25595

An issue was discovered in Xen through 4.14.x. The PCI passthrough code
improperly uses register data. Code paths in Xen’s MSI handling have been
identified that act on unsanitized values read back from device hardware
registers. While devices strictly compliant with PCI specifications
shouldn’t be able to affect these registers, experience shows that it’s
very common for devices to have out-of-spec “backdoor” operations that can
affect the result of these reads. A not fully trusted guest may be able to
crash Xen, leading to a Denial of Service (DoS) for the entire system.
Privilege escalation and information leaks cannot be excluded. All versions
of Xen supporting PCI passthrough are affected. Only x86 systems are
vulnerable. Arm systems are not vulnerable. Only guests with passed through
PCI devices may be able to leverage the vulnerability. Only systems passing
through devices with out-of-spec (“backdoor”) functionality can cause
issues. Experience shows that such out-of-spec functionality is common;
unless you have reason to believe that your device does not have such
functionality, it’s better to assume that it does.

Notes