Re: [PATCH] x86/efi: Access EFI MMIO data as unencrypted when SEV is active

[Ard Biesheuvel]

On 3 July 2018 at 15:32, Brijesh Singh <brijesh.singh@xxxxxxx> wrote:
> SEV guest fails to update the UEFI runtime variables stored in the
> flash. commit 1379edd59673 ("x86/efi: Access EFI data as encrypted
> when SEV is active") unconditionally maps all the UEFI runtime data
> as 'encrypted' (C=1). When SEV is active the UEFI runtime data marked
> as EFI_MEMORY_MAPPED_IO should be mapped as 'unencrypted' so that both
> guest and hypervisor can access the data.
>
> Fixes: 1379edd59673 (x86/efi: Access EFI data as encrypted ...)
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: linux-efi@xxxxxxxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.15.x
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/platform/efi/efi_64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index 77873ce..5f2eb32 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -417,7 +417,7 @@ static void __init __map_region(efi_memory_desc_t *md, u64 va)
>         if (!(md->attribute & EFI_MEMORY_WB))
>                 flags |= _PAGE_PCD;
>
> -       if (sev_active())
> +       if (sev_active() && md->type != EFI_MEMORY_MAPPED_IO)
>                 flags |= _PAGE_ENC;
>
>         pfn = md->phys_addr >> PAGE_SHIFT;

Is it safe to only update this occurrence and not the one in
efi_runtime_update_mappings() ?