Re: [PATCH v8 3/4] efi: Load efi_secret module if EFI secret area is populated

[Dov Murik <dovmurik@linux.ibm.com>]

On 28/02/2022 14:49, Ard Biesheuvel wrote:
> On Mon, 28 Feb 2022 at 12:43, Dov Murik <dovmurik@linux.ibm.com> wrote:
>>
>> If the efi_secret module is built, register a late_initcall in the EFI
>> driver which checks whether the EFI secret area is available and
>> populated, and then requests to load the efi_secret module.
>>
>> This will cause the <securityfs>/secrets/coco directory to appear in
>> guests into which secrets were injected; in other cases, the module is
>> not loaded.
>>
>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
>> Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
> 
> It would be better to simply expose a platform device and associated
> driver, instead of hooking into the module machinery directly.
> 
> We already do something similar for the EFI rtc and the efivars
> subsystem, using platform_device_register_simple()
> 

Thanks Ard, I'll look into this.

I hope the mechanism you suggest allows me to perform complex check to
see if the device is really there (in this case: check for an efi
variable, map memory as encrypted, verify it starts with a specific GUID
-- everything before request_module() in the code below).

-Dov


> 
>> ---
>>  drivers/firmware/efi/Makefile        |  1 +
>>  drivers/firmware/efi/coco.c          | 58 ++++++++++++++++++++
>>  drivers/virt/coco/efi_secret/Kconfig |  3 +
>>  3 files changed, 62 insertions(+)
>>
>> diff --git a/drivers/firmware/efi/Makefile b/drivers/firmware/efi/Makefile
>> index c02ff25dd477..49c4a8c0bfc4 100644
>> --- a/drivers/firmware/efi/Makefile
>> +++ b/drivers/firmware/efi/Makefile
>> @@ -32,6 +32,7 @@ obj-$(CONFIG_APPLE_PROPERTIES)                += apple-properties.o
>>  obj-$(CONFIG_EFI_RCI2_TABLE)           += rci2-table.o
>>  obj-$(CONFIG_EFI_EMBEDDED_FIRMWARE)    += embedded-firmware.o
>>  obj-$(CONFIG_LOAD_UEFI_KEYS)           += mokvar-table.o
>> +obj-$(CONFIG_EFI_COCO_SECRET)          += coco.o
>>
>>  fake_map-y                             += fake_mem.o
>>  fake_map-$(CONFIG_X86)                 += x86_fake_mem.o
>> diff --git a/drivers/firmware/efi/coco.c b/drivers/firmware/efi/coco.c
>> new file mode 100644
>> index 000000000000..f8efd240ab05
>> --- /dev/null
>> +++ b/drivers/firmware/efi/coco.c
>> @@ -0,0 +1,58 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Confidential computing (coco) secret area handling
>> + *
>> + * Copyright (C) 2021 IBM Corporation
>> + * Author: Dov Murik <dovmurik@linux.ibm.com>
>> + */
>> +
>> +#define pr_fmt(fmt) "efi: " fmt
>> +
>> +#include <linux/efi.h>
>> +#include <linux/init.h>
>> +#include <linux/io.h>
>> +#include <linux/kmod.h>
>> +
>> +#ifdef CONFIG_EFI_SECRET_MODULE
>> +
>> +/*
>> + * Load the efi_secret module if the EFI secret area is populated
>> + */
>> +static int __init load_efi_secret_module(void)
>> +{
>> +       struct linux_efi_coco_secret_area *area;
>> +       efi_guid_t *header_guid;
>> +       int ret = 0;
>> +
>> +       if (efi.coco_secret == EFI_INVALID_TABLE_ADDR)
>> +               return 0;
>> +
>> +       area = memremap(efi.coco_secret, sizeof(*area), MEMREMAP_WB);
>> +       if (!area) {
>> +               pr_err("Failed to map confidential computing secret area descriptor\n");
>> +               return -ENOMEM;
>> +       }
>> +       if (!area->base_pa || area->size < sizeof(*header_guid))
>> +               goto unmap_desc;
>> +
>> +       header_guid = (void __force *)ioremap_encrypted(area->base_pa, sizeof(*header_guid));
>> +       if (!header_guid) {
>> +               pr_err("Failed to map secret area\n");
>> +               ret = -ENOMEM;
>> +               goto unmap_desc;
>> +       }
>> +       if (efi_guidcmp(*header_guid, EFI_SECRET_TABLE_HEADER_GUID))
>> +               goto unmap_encrypted;
>> +
>> +       ret = request_module("efi_secret");
>> +
>> +unmap_encrypted:
>> +       iounmap((void __iomem *)header_guid);
>> +
>> +unmap_desc:
>> +       memunmap(area);
>> +       return ret;
>> +}
>> +late_initcall(load_efi_secret_module);
>> +
>> +#endif
>> diff --git a/drivers/virt/coco/efi_secret/Kconfig b/drivers/virt/coco/efi_secret/Kconfig
>> index 4404d198f3b2..dc8da2921e36 100644
>> --- a/drivers/virt/coco/efi_secret/Kconfig
>> +++ b/drivers/virt/coco/efi_secret/Kconfig
>> @@ -14,3 +14,6 @@ config EFI_SECRET
>>
>>           To compile this driver as a module, choose M here.
>>           The module will be called efi_secret.
>> +
>> +         The module is loaded automatically by the EFI driver if the EFI
>> +         secret area is populated.
>> --
>> 2.25.1
>>