CVE-2020-25598

An issue was discovered in Xen 4.14.x. There is a missing unlock in the
XENMEM_acquire_resource error path. The RCU (Read, Copy, Update) mechanism
is a synchronisation primitive. A buggy error path in the
XENMEM_acquire_resource exits without releasing an RCU reference, which is
conceptually similar to forgetting to unlock a spinlock. A buggy or
malicious HVM stubdomain can cause an RCU reference to be leaked. This
causes subsequent administration operations, (e.g., CPU offline) to
livelock, resulting in a host Denial of Service. The buggy codepath has
been present since Xen 4.12. Xen 4.14 and later are vulnerable to the DoS.
The side effects are believed to be benign on Xen 4.12 and 4.13, but
patches are provided nevertheless. The vulnerability can generally only be
exploited by x86 HVM VMs, as these are generally the only type of VM that
have a Qemu stubdomain. x86 PV and PVH domains, as well as ARM VMs,
typically don’t use a stubdomain. Only VMs using HVM stubdomains can
exploit the vulnerability. VMs using PV stubdomains, or with emulators
running in dom0, cannot exploit the vulnerability.

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