Re: [PATCH v2 11/14] x86/alt: Adjust _alternative_instructions() to not create shadow stacks

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.05.2020 21:18, Andrew Cooper wrote:
> @@ -398,6 +399,19 @@ static void __init _alternative_instructions(bool force)
>          panic("Timed out waiting for alternatives self-NMI to hit\n");
>  
>      set_nmi_callback(saved_nmi_callback);
> +
> +    /*
> +     * When Xen is using shadow stacks, the alternatives clearing CR0.WP and
> +     * writing into the mappings set dirty bits, turning the mappings into
> +     * shadow stack mappings.
> +     *
> +     * While we can execute from them, this would also permit them to be the
> +     * target of WRSS instructions, so reset the dirty after patching.
> +     */
> +    if ( cpu_has_xen_shstk )
> +        modify_xen_mappings(XEN_VIRT_START + MB(2),
> +                            (unsigned long)&__2M_text_end,
> +                            PAGE_HYPERVISOR_RX);

Am I misremembering, or did you post a patch before that should
be part of this series, as being a prereq to this change,
making modify_xen_mappings() also respect _PAGE_DIRTY as
requested by the caller?

Additionally I notice this

        if ( desc->Attribute & (efi_bs_revision < EFI_REVISION(2, 5)
                                ? EFI_MEMORY_WP : EFI_MEMORY_RO) )
            prot &= ~_PAGE_RW;

in efi_init_memory(). Afaict we will need to clear _PAGE_DIRTY
there as well, with prot starting out as PAGE_HYPERVISOR_RWX.

Jan