Re: [PATCH v9 6/6] x86/efi: Safely enable unaccepted memory in UEFI

[Ard Biesheuvel]

On Tue, 6 Jun 2023 at 16:52, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> From: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
>
> The UEFI v2.9 specification includes a new memory type to be used in
> environments where the OS must accept memory that is provided from its
> host. Before the introduction of this memory type, all memory was
> accepted eagerly in the firmware. In order for the firmware to safely
> stop accepting memory on the OS's behalf, the OS must affirmatively
> indicate support to the firmware. This is only a problem for AMD
> SEV-SNP, since Linux has had support for it since 5.19. The other
> technology that can make use of unaccepted memory, Intel TDX, does not
> yet have Linux support, so it can strictly require unaccepted memory
> support as a dependency of CONFIG_TDX and not require communication with
> the firmware.
>
> Enabling unaccepted memory requires calling a 0-argument enablement
> protocol before ExitBootServices. This call is only made if the kernel
> is compiled with UNACCEPTED_MEMORY=y
>
> This protocol will be removed after the end of life of the first LTS
> that includes it, in order to give firmware implementations an
> expiration date for it. When the protocol is removed, firmware will
> strictly infer that a SEV-SNP VM is running an OS that supports the
> unaccepted memory type. At the earliest convenience, when unaccepted
> memory support is added to Linux, SEV-SNP may take strict dependence in
> it. After the firmware removes support for the protocol, this patch
> should be reverted.
>
>   [tl: address some checkscript warnings]
>
> Signed-off-by: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> ---
>  drivers/firmware/efi/libstub/x86-stub.c | 36 +++++++++++++++++++++++++
>  include/linux/efi.h                     |  3 +++
>  2 files changed, 39 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index 8d17cee8b98e..e2193dbe1f66 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -26,6 +26,17 @@ const efi_dxe_services_table_t *efi_dxe_table;
>  u32 image_offset __section(".data");
>  static efi_loaded_image_t *image = NULL;
>
> +typedef union sev_memory_acceptance_protocol sev_memory_acceptance_protocol_t;
> +union sev_memory_acceptance_protocol {
> +       struct {
> +               efi_status_t (__efiapi * allow_unaccepted_memory)(
> +                       sev_memory_acceptance_protocol_t *);
> +       };
> +       struct {
> +               u32 allow_unaccepted_memory;
> +       } mixed_mode;
> +};
> +
>  static efi_status_t
>  preserve_pci_rom_image(efi_pci_io_protocol_t *pci, struct pci_setup_rom **__rom)
>  {
> @@ -310,6 +321,29 @@ setup_memory_protection(unsigned long image_base, unsigned long image_size)
>  #endif
>  }
>
> +static void setup_unaccepted_memory(void)
> +{
> +       efi_guid_t mem_acceptance_proto = OVMF_SEV_MEMORY_ACCEPTANCE_PROTOCOL_GUID;
> +       sev_memory_acceptance_protocol_t *proto;
> +       efi_status_t status;
> +
> +       if (!IS_ENABLED(CONFIG_UNACCEPTED_MEMORY))
> +               return;
> +
> +       /*
> +        * Enable unaccepted memory before calling exit boot services in order
> +        * for the UEFI to not accept all memory on EBS.
> +        */
> +       status = efi_bs_call(locate_protocol, &mem_acceptance_proto, NULL,
> +                            (void **)&proto);
> +       if (status != EFI_SUCCESS)
> +               return;
> +
> +       status = efi_call_proto(proto, allow_unaccepted_memory);
> +       if (status != EFI_SUCCESS)
> +               efi_err("Memory acceptance protocol failed\n");
> +}
> +
>  static const efi_char16_t apple[] = L"Apple";
>
>  static void setup_quirks(struct boot_params *boot_params,
> @@ -908,6 +942,8 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>
>         setup_quirks(boot_params, bzimage_addr, buffer_end - buffer_start);
>
> +       setup_unaccepted_memory();
> +
>         status = exit_boot(boot_params, handle);
>         if (status != EFI_SUCCESS) {
>                 efi_err("exit_boot() failed!\n");
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 9864f9c00da2..8c5abcf70a05 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -437,6 +437,9 @@ void efi_native_runtime_setup(void);
>  #define DELLEMC_EFI_RCI2_TABLE_GUID            EFI_GUID(0x2d9f28a2, 0xa886, 0x456a,  0x97, 0xa8, 0xf1, 0x1e, 0xf2, 0x4f, 0xf4, 0x55)
>  #define AMD_SEV_MEM_ENCRYPT_GUID               EFI_GUID(0x0cf29b71, 0x9e51, 0x433a,  0xa3, 0xb7, 0x81, 0xf3, 0xab, 0x16, 0xb8, 0x75)
>
> +/* OVMF protocol GUIDs */
> +#define OVMF_SEV_MEMORY_ACCEPTANCE_PROTOCOL_GUID       EFI_GUID(0xc5a010fe, 0x38a7, 0x4531,  0x8a, 0x4a, 0x05, 0x00, 0xd2, 0xfd, 0x16, 0x49)
> +
>  typedef struct {
>         efi_guid_t guid;
>         u64 table;
> --
> 2.40.1
>