CVE-2020-25603

An issue was discovered in Xen through 4.14.x. There are missing memory
barriers when accessing/allocating an event channel. Event channels control
structures can be accessed lockless as long as the port is considered to be
valid. Such a sequence is missing an appropriate memory barrier (e.g.,
smp_*mb()) to prevent both the compiler and CPU from re-ordering access. A
malicious guest may be able to cause a hypervisor crash resulting in a
Denial of Service (DoS). Information leak and privilege escalation cannot
be excluded. Systems running all versions of Xen are affected. Whether a
system is vulnerable will depend on the CPU and compiler used to build Xen.
For all systems, the presence and the scope of the vulnerability depend on
the precise re-ordering performed by the compiler used to build Xen. We
have not been able to survey compilers; consequently we cannot say which
compiler(s) might produce vulnerable code (with which code generation
options). GCC documentation clearly suggests that re-ordering is possible.
Arm systems will also be vulnerable if the CPU is able to re-order memory
access. Please consult your CPU vendor. x86 systems are only vulnerable if
a compiler performs re-ordering.

Notes