Re: [PATCH v2 06/16] x86/efi: Generating random HMAC key for siging hibernate image

Matt Fleming
2015-08-20T20:40:56Z

On Tue, 11 Aug, at 02:16:26PM, Lee, Chun-Yi wrote:
> This patch adds codes in EFI stub for generating and storing the
> HMAC key in EFI boot service variable for signing hibernate image.
>
> Per rcf2104, the length of HMAC-SHA1 hash result is 20 bytes, and
> it recommended the length of key the same with hash rsult, means
> also 20 bytes. Using longer key would not significantly increase
> the function strength. Due to the nvram space is limited in some
> UEFI machines, so using the minimal recommended length 20 bytes
> key that will stored in boot service variable.

I'm having a hard time understanding the middle part of this
paragraph, specifically the part of the key and the hash result.
There's a typo in the subject too s/siging/signing/and "Generating"
should be "Generate".

> The HMAC key stored in EFI boot service variable, GUID is
> HIBERNATIONKey-fe141863-c070-478e-b8a3-878a5dc9ef21.

I'd really like to see some of the explanation from your cover letter
included in the commit message for this patch, and in particular why
signing hibernate images is a good thing.

Recording that for posterity in the commit message is going to be
helpful when someone looks at this patch in 2 years time and wonders
why RNG support was added to the EFI stub and why they might want to
sign hibernate images.

> Reviewed-by: Jiri Kosina <jkosina@suse.com>
> Tested-by: Jiri Kosina <jkosina@suse.com>
> Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
> ---
> arch/x86/boot/compressed/eboot.c | 60 ++++++++++++++++++++++++++++++++++++++++
> arch/x86/include/asm/suspend.h | 9 ++++++
> include/linux/suspend.h | 3 ++
> 3 files changed, 72 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
> index 0ffb6db..463aa9b 100644
> --- a/arch/x86/boot/compressed/eboot.c
> +++ b/arch/x86/boot/compressed/eboot.c
> @@ -12,6 +12,7 @@
> #include <asm/efi.h>
> #include <asm/setup.h>
> #include <asm/desc.h>
> +#include <asm/suspend.h>
>
> #include "../string.h"
> #include "eboot.h"
> @@ -1383,6 +1384,63 @@ free_mem_map:
> return status;
> }
>
> +#ifdef CONFIG_HIBERNATE_VERIFICATION
> +#define HIBERNATION_KEY \
> + ((efi_char16_t [15]) { 'H', 'I', 'B', 'E', 'R', 'N', 'A', 'T', 'I', 'O', 'N', 'K', 'e', 'y', 0 })
> +#define HIBERNATION_KEY_ATTRIBUTE (EFI_VARIABLE_NON_VOLATILE | \
> + EFI_VARIABLE_BOOTSERVICE_ACCESS)
> +
> +static void setup_hibernation_keys(struct boot_params *params)
> +{
> + unsigned long key_size;
> + unsigned long attributes;
> + struct hibernation_keys *keys;
> + efi_status_t status;
> +
> + /* Allocate setup_data to carry keys */
> + status = efi_call_early(allocate_pool, EFI_LOADER_DATA,
> + sizeof(struct hibernation_keys), &keys);
> + if (status != EFI_SUCCESS) {
> + efi_printk(sys_table, "Failed to alloc mem for hibernation keys\n");
> + return;
> + }
> +
> + memset(keys, 0, sizeof(struct hibernation_keys));
> +
> + status = efi_call_early(get_variable, HIBERNATION_KEY,
> + &EFI_HIBERNATION_GUID, &attributes,
> + &key_size, keys->hibernation_key);

Tiny nit, but could you put a new line here please? This is a large
chunk of code.

> + if (status == EFI_SUCCESS && attributes != HIBERNATION_KEY_ATTRIBUTE) {
> + efi_printk(sys_table, "A hibernation key is not boot service variable\n");
> + memset(keys->hibernation_key, 0, HIBERNATION_DIGEST_SIZE);
> + status = efi_call_early(set_variable, HIBERNATION_KEY,
> + &EFI_HIBERNATION_GUID, attributes, 0,
> + NULL);
> + if (status == EFI_SUCCESS) {
> + efi_printk(sys_table, "Cleaned existing hibernation key\n");
> + status = EFI_NOT_FOUND;
> + }
> + }

Hmm.. it's not clear to me that we should be deleting this EFI
variable if the attributes are bogus. It would be safer to just bail.

> +
> + if (status != EFI_SUCCESS) {
> + efi_printk(sys_table, "Failed to get existing hibernation key\n");
> +
> + efi_get_random_key(sys_table, params, keys->hibernation_key,
> + HIBERNATION_DIGEST_SIZE);
> +
> + status = efi_call_early(set_variable, HIBERNATION_KEY,
> + &EFI_HIBERNATION_GUID,
> + HIBERNATION_KEY_ATTRIBUTE,
> + HIBERNATION_DIGEST_SIZE,
> + keys->hibernation_key);
> + if (status != EFI_SUCCESS)
> + efi_printk(sys_table, "Failed to set hibernation key\n");

You're leaking 'keys' here.

> diff --git a/arch/x86/include/asm/suspend.h b/arch/x86/include/asm/suspend.h
> index 2fab6c2..ab463c4 100644
> --- a/arch/x86/include/asm/suspend.h
> +++ b/arch/x86/include/asm/suspend.h
> @@ -3,3 +3,12 @@
> #else
> # include <asm/suspend_64.h>
> #endif
> +
> +#ifdef CONFIG_HIBERNATE_VERIFICATION
> +#include <linux/suspend.h>
> +
> +struct hibernation_keys {
> + unsigned long hkey_status;
> + u8 hibernation_key[HIBERNATION_DIGEST_SIZE];
> +};
> +#endif

Have you given any thought to how things are going to work if we
change the hash function in the future, or provide a choice? That
information doesn't appear anywhere in the above struct.

--
Matt Fleming, Intel Open Source Technology Center