Re: [PATCH V40 12/29] x86: Lock down IO port access when the kernel is locked down

On Wed, Jan 5, 2022 at 2:48 PM Matthew Garrett [off-list ref] wrote:

On Wed, Jan 05, 2022 at 02:25:41PM +0800, Kai-Heng Feng wrote:

quoted

This patch breaks ioperm() usage from userspace programs with CAP_SYS_RAWIO cap.

I wonder if it's possible to revert this commit?

When lockdown is enabled, or under all circumstances? It's expected to
be blocked when lockdown is enabled - allowing userland to use port IO
would potentially allow reconfiguration of PCI devices in ways that
could alter kernel behaviour in ways relevant to security, which is what
lockdown aims to prevent. What's being broken by this?

Only when lockdown is enabled.

The affected system from the customer has SecureBoot enabled (and
hence lockdown), and the kernel upgrade surprisingly broke ioperm()
usage.
The userspace program is proprietary so I can't share it here.

Basically this patch makes ioperm() a noop on SecureBoot enabled x86 systems.
If reverting is not an option, what else can we do to circumvent the regression?

Kai-Heng