Re: [PATCH 5/5] x86/HVM: improve CET-IBT pruning of ENDBR

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 16, 2023 at 02:33:14PM +0100, Jan Beulich wrote:
> __init{const,data}_cf_clobber can have an effect only for pointers
> actually populated in the respective tables. While not the case for SVM
> right now, VMX installs a number of pointers only under certain
> conditions. Hence the respective functions would have their ENDBR purged
> only when those conditions are met. Invoke "pruning" functions after
> having copied the respective tables, for them to install any "missing"
> pointers.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> This is largely cosmetic for present hardware, which when supporting
> CET-IBT likely also supports all of the advanced VMX features for which
> hook pointers are installed conditionally. The only case this would make
> a difference there is when use of respective features was suppressed via
> command line option (where available). For future hooks it may end up
> relevant even by default, and it also would be if AMD started supporting
> CET-IBT; right now it matters only for .pi_update_irte, as iommu_intpost
> continues to default to off.
> 
> Originally I had meant to put the SVM and VMX functions in presmp-
> initcalls, but hvm/{svm,vmx}/built_in.o are linked into hvm/built_in.o
> before hvm/hvm.o. And I don't think I want to fiddle with link order
> here.
> 
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -161,10 +161,15 @@ static int __init cf_check hvm_enable(vo
>      else if ( cpu_has_svm )
>          fns = start_svm();
>  
> +    if ( fns )
> +        hvm_funcs = *fns;
> +
> +    prune_vmx();
> +    prune_svm();
> +
>      if ( fns == NULL )
>          return 0;
>  
> -    hvm_funcs = *fns;
>      hvm_enabled = 1;
>  
>      printk("HVM: %s enabled\n", fns->name);
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -2587,6 +2587,19 @@ const struct hvm_function_table * __init
>      return &svm_function_table;
>  }
>  
> +void __init prune_svm(void)
> +{
> +    /*
> +     * Now that svm_function_table was copied, populate all function pointers
> +     * which may have been left at NULL, for __initdata_cf_clobber to have as
> +     * much of an effect as possible.
> +     */
> +    if ( !IS_ENABLED(CONFIG_XEN_IBT) )

Shouldn't this better use cpu_has_xen_ibt?

Otherwise the clobbering done in _apply_alternatives() won't be
engaged, so it's pointless to set the extra fields.

> +        return;
> +
> +    /* Nothing at present. */
> +}
> +
>  void svm_vmexit_handler(void)
>  {
>      struct cpu_user_regs *regs = guest_cpu_user_regs();
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -3032,6 +3032,30 @@ const struct hvm_function_table * __init
>      return &vmx_function_table;
>  }
>  
> +void __init prune_vmx(void)
> +{
> +    /*
> +     * Now that vmx_function_table was copied, populate all function pointers
> +     * which may have been left at NULL, for __initdata_cf_clobber to have as
> +     * much of an effect as possible.
> +     */
> +    if ( !IS_ENABLED(CONFIG_XEN_IBT) )
> +        return;
> +
> +    vmx_function_table.set_descriptor_access_exiting =
> +        vmx_set_descriptor_access_exiting;
> +
> +    vmx_function_table.update_eoi_exit_bitmap = vmx_update_eoi_exit_bitmap;
> +    vmx_function_table.process_isr            = vmx_process_isr;
> +    vmx_function_table.handle_eoi             = vmx_handle_eoi;
> +
> +    vmx_function_table.pi_update_irte = vmx_pi_update_irte;
> +
> +    vmx_function_table.deliver_posted_intr = vmx_deliver_posted_intr;
> +    vmx_function_table.sync_pir_to_irr     = vmx_sync_pir_to_irr;
> +    vmx_function_table.test_pir            = vmx_test_pir;

Hm, I find this quite fragile, as it's easy to add a new handler
without realizing that addition here might also be required.

I don't really have good ideas about how to handle this, unless we
populate unused handlers with some poison and loop over the structure
as an array of pointers and choke on finding one of them pointing to
NULL.

Thanks, Roger.