Re: [PATCH v2 2/2] x86: Call Shim Verify in the multiboot2 path

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2024 16:11, Ross Lagerwall wrote:
> --- a/xen/arch/x86/efi/efi-boot.h
> +++ b/xen/arch/x86/efi/efi-boot.h
> @@ -3,6 +3,7 @@
>   * is intended to be included by common/efi/boot.c _only_, and
>   * therefore can define arch specific global variables.
>   */
> +#include <xen/multiboot2.h>
>  #include <xen/vga.h>
>  #include <asm/e820.h>
>  #include <asm/edd.h>
> @@ -808,9 +809,69 @@ static const char *__init get_option(const char *cmd, 
> const char *opt)
>      return o;
>  }
>  
> +#define ALIGN_UP(arg, align) \
> +                (((arg) + (align) - 1) & ~((typeof(arg))(align) - 1))

Nit: I don't think aligning the opening parentheses is an appropriate
criteria here. Imo either

#define ALIGN_UP(arg, align) \
            (((arg) + (align) - 1) & ~((typeof(arg))(align) - 1))

or

#define ALIGN_UP(arg, align) \
        (((arg) + (align) - 1) & ~((typeof(arg))(align) - 1))

or

#define ALIGN_UP(arg, align) \
    (((arg) + (align) - 1) & ~((typeof(arg))(align) - 1))

.

> +static void __init efi_verify_dom0(uint64_t mbi_in)
> +{
> +    uint64_t ptr;
> +    const multiboot2_tag_t *tag;
> +    EFI_SHIM_LOCK_PROTOCOL *shim_lock;
> +    EFI_STATUS status;
> +    const multiboot2_tag_module_t *kernel = NULL;
> +    const multiboot2_fixed_t *mbi_fix = _p(mbi_in);
> +    static EFI_GUID __initdata shim_lock_guid = SHIM_LOCK_PROTOCOL_GUID;
> +    static EFI_GUID __initdata global_variable_guid = EFI_GLOBAL_VARIABLE;
> +
> +    ptr = ALIGN_UP(mbi_in + sizeof(*mbi_fix), MULTIBOOT2_TAG_ALIGN);
> +
> +    for ( tag = _p(ptr); (uint64_t)tag - mbi_in < mbi_fix->total_size;
> +          tag = _p(ALIGN_UP((uint64_t)tag + tag->size, 
> MULTIBOOT2_TAG_ALIGN)) )
> +    {
> +        if ( tag->type == MULTIBOOT2_TAG_TYPE_MODULE )
> +        {
> +            kernel = (const multiboot2_tag_module_t *)tag;
> +            break;

This could do with a comment along the lines of what __start_xen() has
("Dom0 kernel is always first").

> +        }
> +        else if ( tag->type == MULTIBOOT2_TAG_TYPE_END )

Not need for "else" here (personally I find such irritating).

> +            break;
> +    }
> +
> +    if ( !kernel )
> +        return;
> +
> +    if ( (status = efi_bs->LocateProtocol(&shim_lock_guid, NULL,
> +                                          (void **)&shim_lock)) != 
> EFI_SUCCESS )
> +    {
> +        UINT32 attr;
> +        UINT8 data;
> +        UINTN size = sizeof(data);
> +
> +        status = efi_rs->GetVariable((CHAR16 *)L"SecureBoot", 
> &global_variable_guid,
> +                                     &attr, &size, &data);
> +        if ( status == EFI_NOT_FOUND )
> +            return;
> +
> +        if ( EFI_ERROR(status) )
> +            PrintErrMesg(L"Could not get SecureBoot variable", status);
> +
> +        if ( attr != (EFI_VARIABLE_BOOTSERVICE_ACCESS | 
> EFI_VARIABLE_RUNTIME_ACCESS) )
> +            PrintErrMesg(L"Unexpected SecureBoot attributes", attr);

This wants to be blexit(), not PrintErrMesg().

> +        if ( size == 1 && data == 0 )
> +            return;
> +
> +        blexit(L"Could not locate shim but Secure Boot is enabled");
> +    }
> +
> +    if ( (status = shim_lock->Verify(_p(kernel->mod_start),
> +                                     kernel->mod_end - kernel->mod_start)) 
> != EFI_SUCCESS )
> +        PrintErrMesg(L"Dom0 kernel image could not be verified", status);
> +}

Overall this is a superset of what efi_start() does. What I'm missing from
the description is some discussion of why what's done there is not
sufficient (beyond the env var check, which iirc there once was a patch to
add it). One could only then judge whether it wouldn't make sense to make
this function uniformly used by both paths (with mbi_in suitably dealt with
for the other case).

Jan