Linux ioperm bitmap context switching issues

ISSUE DESCRIPTION

Linux 5.5 overhauled the internal state handling for the iopl() and ioperm() system calls. Unfortunately, one aspect on context switch wasn’t wired up correctly for the Xen PVOps case.

IMPACT

IO port permissions don’t get rescinded when context switching to an unprivileged task. Therefore, all userspace can use the IO ports granted to the most recently scheduled task with IO port permissions.

VULNERABLE SYSTEMS

Only x86 guests are vulnerable.
All versions of Linux from 5.5 are potentially vulnerable.
Linux is only vulnerable when running as x86 PV guest. Linux is not vulnerable when running as an x86 HVM/PVH guests.
The vulnerability can only be exploited in domains which have been granted access to IO ports by Xen. This is typically only the hardware domain, and guests configured with PCI Passthrough.