CVE-2019-18425

An issue was discovered in Xen through 4.12.x allowing 32-bit PV guest OS
users to gain guest OS privileges by installing and using descriptors.
There is missing descriptor table limit checking in x86 PV emulation. When
emulating certain PV guest operations, descriptor table accesses are
performed by the emulating code. Such accesses should respect the guest
specified limits, unless otherwise guaranteed to fail in such a case.
Without this, emulation of 32-bit guest user mode calls through call gates
would allow guest user mode to install and then use descriptors of their
choice, as long as the guest kernel did not itself install an LDT. (Most
OSes don’t install any LDT by default). 32-bit PV guest user mode can
elevate its privileges to that of the guest kernel. Xen versions from at
least 3.2 onwards are affected. Only 32-bit PV guest user mode can leverage
this vulnerability. HVM, PVH, as well as 64-bit PV guests cannot leverage
this vulnerability. Arm systems are unaffected.

Notes