Re: [PATCH v4 1/3] efi/libstub: Copy confidential computing secret area

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Oct 20, 2021 at 06:14:06AM +0000, Dov Murik wrote:
> Confidential computing (coco) hardware such as AMD SEV (Secure Encrypted
> Virtualization) allows a guest owner to inject secrets into the VMs
> memory without the host/hypervisor being able to read them.
> 
> Firmware support for secret injection is available in OVMF, which
> reserves a memory area for secret injection and includes a pointer to it
> the in EFI config table entry LINUX_EFI_COCO_SECRET_TABLE_GUID.
> However, OVMF doesn't force the guest OS to keep this memory area
> reserved.
> 
> If EFI exposes such a table entry, efi/libstub will copy this area to a
> reserved memory for future use inside the kernel.
> 
> A pointer to the new copy is kept in the EFI table under
> LINUX_EFI_COCO_SECRET_AREA_GUID.
> 
> The new functionality can be enabled with CONFIG_EFI_COCO_SECRET=y.
> 
> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> ---
>  drivers/firmware/efi/Kconfig            | 12 +++++
>  drivers/firmware/efi/libstub/Makefile   |  1 +
>  drivers/firmware/efi/libstub/coco.c     | 68 +++++++++++++++++++++++++
>  drivers/firmware/efi/libstub/efi-stub.c |  2 +
>  drivers/firmware/efi/libstub/efistub.h  |  6 +++
>  drivers/firmware/efi/libstub/x86-stub.c |  2 +
>  include/linux/efi.h                     |  6 +++
>  7 files changed, 97 insertions(+)
>  create mode 100644 drivers/firmware/efi/libstub/coco.c
> 
> diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
> index 2c3dac5ecb36..68d1c5e6a7b5 100644
> --- a/drivers/firmware/efi/Kconfig
> +++ b/drivers/firmware/efi/Kconfig
> @@ -284,3 +284,15 @@ config EFI_CUSTOM_SSDT_OVERLAYS
>  
>  	  See Documentation/admin-guide/acpi/ssdt-overlays.rst for more
>  	  information.
> +
> +config EFI_COCO_SECRET
> +	bool "Copy and reserve EFI Confidential Computing secret area"
> +	depends on EFI
> +	default n

default is always "n", no need to list this.

> +	help
> +	  Copy memory reserved by EFI for Confidential Computing (coco)
> +	  injected secrets, if EFI exposes such a table entry.

Why would you want to "copy" secret memory?

This sounds really odd here, it sounds like you are opening up a
security hole.  Are you sure this is the correct text that everyone on
the "COCO" group agrees with?

> +
> +	  If you say Y here, the EFI stub copy the EFI secret area (if
> +	  available) and reserve it for use inside the kernel.  This will
> +	  allow the virt/coo/efi_secret module to access the secrets.

What is "virt/coo/efi_secret"?

> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index d0537573501e..fdada3fd5d9b 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -66,6 +66,7 @@ $(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
>  lib-$(CONFIG_EFI_GENERIC_STUB)	+= efi-stub.o fdt.o string.o \
>  				   $(patsubst %.c,lib-%.o,$(efi-deps-y))
>  
> +lib-$(CONFIG_EFI_COCO_SECRET)	+= coco.o
>  lib-$(CONFIG_ARM)		+= arm32-stub.o
>  lib-$(CONFIG_ARM64)		+= arm64-stub.o
>  lib-$(CONFIG_X86)		+= x86-stub.o
> diff --git a/drivers/firmware/efi/libstub/coco.c b/drivers/firmware/efi/libstub/coco.c
> new file mode 100644
> index 000000000000..bf546b6a3f72
> --- /dev/null
> +++ b/drivers/firmware/efi/libstub/coco.c
> @@ -0,0 +1,68 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Confidential computing (coco) secret area handling
> + *
> + * Copyright (C) 2021 IBM Corporation
> + * Author: Dov Murik <dovmurik@linux.ibm.com>
> + */
> +
> +#include <linux/efi.h>
> +#include <linux/sizes.h>
> +#include <asm/efi.h>
> +
> +#include "efistub.h"
> +
> +#define LINUX_EFI_COCO_SECRET_TABLE_GUID                                                           \
> +	EFI_GUID(0xadf956ad, 0xe98c, 0x484c, 0xae, 0x11, 0xb5, 0x1c, 0x7d, 0x33, 0x64, 0x47)
> +
> +/**
> + * struct efi_coco_secret_table - EFI config table that points to the
> + * confidential computing secret area. The guid
> + * LINUX_EFI_COCO_SECRET_TABLE_GUID holds this table.
> + * @base:	Physical address of the EFI secret area
> + * @size:	Size (in bytes) of the EFI secret area
> + */
> +struct efi_coco_secret_table {
> +	u64 base;
> +	u64 size;

__le64?  Or is this really in host endian format?

> +} __attribute((packed));
> +
> +/*
> + * Create a copy of EFI's confidential computing secret area (if available) so
> + * that the secrets are accessible in the kernel after ExitBootServices.
> + */
> +void efi_copy_coco_secret_area(void)
> +{
> +	efi_guid_t linux_secret_area_guid = LINUX_EFI_COCO_SECRET_AREA_GUID;
> +	efi_status_t status;
> +	struct efi_coco_secret_table *secret_table;
> +	struct linux_efi_coco_secret_area *secret_area;
> +
> +	secret_table = get_efi_config_table(LINUX_EFI_COCO_SECRET_TABLE_GUID);
> +	if (!secret_table)
> +		return;
> +
> +	if (secret_table->size == 0 || secret_table->size >= SZ_4G)
> +		return;
> +
> +	/* Allocate space for the secret area and copy it */
> +	status = efi_bs_call(allocate_pool, EFI_LOADER_DATA,
> +			     sizeof(*secret_area) + secret_table->size, (void **)&secret_area);
> +
> +	if (status != EFI_SUCCESS) {
> +		efi_err("Unable to allocate memory for confidential computing secret area copy\n");
> +		return;
> +	}
> +
> +	secret_area->size = secret_table->size;
> +	memcpy(secret_area->area, (void *)(unsigned long)secret_table->base, secret_table->size);

Why the double cast?

And you can treat this value as a "raw" pointer directly?  No need to
map it at all?  What could go wrong...

> +
> +	status = efi_bs_call(install_configuration_table, &linux_secret_area_guid, secret_area);
> +	if (status != EFI_SUCCESS)
> +		goto err_free;
> +
> +	return;
> +
> +err_free:
> +	efi_bs_call(free_pool, secret_area);

This memory is never freed when shutting down the system?

thanks,

greg k-h