Re: [PATCHv9 04/16] x86/cpu: Defer CR pinning setup until core initcall

["H. Peter Anvin" <hpa@zytor.com>]

On August 2, 2025 11:51:28 AM PDT, Kees Cook <kees@kernel.org> wrote:
>On Thu, Jul 31, 2025 at 05:01:37PM -0700, Dave Hansen wrote:
>> On 7/31/25 16:45, Sohil Mehta wrote:
>> > On 7/9/2025 10:00 AM, Dave Hansen wrote:
>> >> On 7/7/25 01:03, Kirill A. Shutemov wrote:
>> >>> Instead of moving setup_cr_pinning() below efi_enter_virtual_mode() in
>> >>> arch_cpu_finalize_init(), defer it until core initcall.
>> >> What are the side effects of this move? Are there other benefits? What
>> >> are the risks?
>> >>
>> > Picking this up from Kirill.. Reevaluating this, core_initcall() seems
>> > too late for setup_cr_pinning().
>> > 
>> > We need to have CR pinning completed, and the associated static key
>> > enabled before AP bring up. start_secondary()->cr4_init() depends on the
>> > cr_pinning static key to initialize CR4 for APs.
>> 
>> Sure, if you leave cr4_init() completely as-is.
>> 
>> 'cr4_pinned_bits' should be set by the boot CPU. Secondary CPUs should
>> also read 'cr4_pinned_bits' when setting up their own cr4's,
>> unconditionally, independent of 'cr_pinning'.
>> 
>> The thing I think we should change is the pinning _enforcement_. The
>> easiest way to do that is to remove the static_branch_likely() in
>> cr4_init() and then delay flipping the static branch until just before
>> userspace starts.
>
>Yeah, this is fine from my perspective. The goal with the pinning was
>about keeping things safe in the face of an attack from userspace that
>managed to get at MSR values and keeping them from being trivially
>changed.
>

I have mentioned this before: I would like to see CR4-pinning use a patchable immediate to make it harder to manipulate. If the mask is final when alternatives are run, that would be a good time to install it; the code can just contain a zero immediate (no pinning) or a very limited set of bits that must never be changed at all up to that point.