[PATCH v2 3/6] efi_loader: don't load signature database from file

[Heinrich Schuchardt]

On 8/27/21 6:49 AM, AKASHI Takahiro wrote:
> On Fri, Aug 27, 2021 at 06:42:39AM +0200, Heinrich Schuchardt wrote:
>> On 8/27/21 6:12 AM, AKASHI Takahiro wrote:
>>> On Thu, Aug 26, 2021 at 03:48:02PM +0200, Heinrich Schuchardt wrote:
>>>> The UEFI specification requires that the signature database may only be
>>>> stored in tamper-resistant storage. So these variable may not be read
>>>> from an unsigned file.
>>>
>>> I don't have a strong opinion here, but it seems to be too restrictive.
>>> Nobody expects that file-based variable implementation is *safe*.
>>> Leave it as it is so that people can easily experiment secure boot.
>>
>> If the prior boot stage checks the integrity of the U-Boot binary, the
>> file based implementation becomes 'safe' with this patch.
>
> How safe (or secure) is it? That is a question.
> What is your thread model?

The preseed store is as safe as the capsule updates that Linaro is
working on where the certificate for verifying the capsule is baked into
U-Boot or the StMM based variables.

They all require that an attacker can neither load a manipulated U-Boot
nor that he can alter the memory containing U-Boot at runtime.

Best regards

Heinrich

>
> -Takahiro Akashi
>
>
>> Users can still experiment with secure boot by setting the secure boot
>> variables via the efidebug command.
>>
>> I cannot see a use case for having the secure boot data base on an
>> insecure medium.
>>
>> Best regards
>>
>> Heinrich
>>
>>>
>>> -Takahiro Akashi
>>>
>>>
>>>> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
>>>> ---
>>>> v2:
>>>> 	no change
>>>> ---
>>>>    include/efi_variable.h          |  5 +++-
>>>>    lib/efi_loader/efi_var_common.c |  2 --
>>>>    lib/efi_loader/efi_var_file.c   | 41 ++++++++++++++++++++-------------
>>>>    lib/efi_loader/efi_variable.c   |  2 +-
>>>>    4 files changed, 30 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/include/efi_variable.h b/include/efi_variable.h
>>>> index 4623a64142..2d97655e1f 100644
>>>> --- a/include/efi_variable.h
>>>> +++ b/include/efi_variable.h
>>>> @@ -161,10 +161,13 @@ efi_status_t __maybe_unused efi_var_collect(struct efi_var_file **bufp, loff_t *
>>>>    /**
>>>>     * efi_var_restore() - restore EFI variables from buffer
>>>>     *
>>>> + * Only if @safe is set secure boot related variables will be restored.
>>>> + *
>>>>     * @buf:	buffer
>>>> + * @safe:	restoring from tamper-resistant storage
>>>>     * Return:	status code
>>>>     */
>>>> -efi_status_t efi_var_restore(struct efi_var_file *buf);
>>>> +efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe);
>>>>
>>>>    /**
>>>>     * efi_var_from_file() - read variables from file
>>>> diff --git a/lib/efi_loader/efi_var_common.c b/lib/efi_loader/efi_var_common.c
>>>> index cf7afecd60..b0c5b672c5 100644
>>>> --- a/lib/efi_loader/efi_var_common.c
>>>> +++ b/lib/efi_loader/efi_var_common.c
>>>> @@ -32,10 +32,8 @@ static const struct efi_auth_var_name_type name_type[] = {
>>>>    	{u"KEK", &efi_global_variable_guid, EFI_AUTH_VAR_KEK},
>>>>    	{u"db",  &efi_guid_image_security_database, EFI_AUTH_VAR_DB},
>>>>    	{u"dbx",  &efi_guid_image_security_database, EFI_AUTH_VAR_DBX},
>>>> -	/* not used yet
>>>>    	{u"dbt",  &efi_guid_image_security_database, EFI_AUTH_VAR_DBT},
>>>>    	{u"dbr",  &efi_guid_image_security_database, EFI_AUTH_VAR_DBR},
>>>> -	*/
>>>>    };
>>>>
>>>>    static bool efi_secure_boot;
>>>> diff --git a/lib/efi_loader/efi_var_file.c b/lib/efi_loader/efi_var_file.c
>>>> index de076b8cbc..c7c6805ed0 100644
>>>> --- a/lib/efi_loader/efi_var_file.c
>>>> +++ b/lib/efi_loader/efi_var_file.c
>>>> @@ -148,9 +148,10 @@ error:
>>>>    #endif
>>>>    }
>>>>
>>>> -efi_status_t efi_var_restore(struct efi_var_file *buf)
>>>> +efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe)
>>>>    {
>>>>    	struct efi_var_entry *var, *last_var;
>>>> +	u16 *data;
>>>>    	efi_status_t ret;
>>>>
>>>>    	if (buf->reserved || buf->magic != EFI_VAR_FILE_MAGIC ||
>>>> @@ -160,21 +161,29 @@ efi_status_t efi_var_restore(struct efi_var_file *buf)
>>>>    		return EFI_INVALID_PARAMETER;
>>>>    	}
>>>>
>>>> -	var = buf->var;
>>>>    	last_var = (struct efi_var_entry *)((u8 *)buf + buf->length);
>>>> -	while (var < last_var) {
>>>> -		u16 *data = var->name + u16_strlen(var->name) + 1;
>>>> -
>>>> -		if (var->attr & EFI_VARIABLE_NON_VOLATILE && var->length) {
>>>> -			ret = efi_var_mem_ins(var->name, &var->guid, var->attr,
>>>> -					      var->length, data, 0, NULL,
>>>> -					      var->time);
>>>> -			if (ret != EFI_SUCCESS)
>>>> -				log_err("Failed to set EFI variable %ls\n",
>>>> -					var->name);
>>>> -		}
>>>> -		var = (struct efi_var_entry *)
>>>> -		      ALIGN((uintptr_t)data + var->length, 8);
>>>> +	for (var = buf->var; var < last_var;
>>>> +	     var = (struct efi_var_entry *)
>>>> +		   ALIGN((uintptr_t)data + var->length, 8)) {
>>>> +
>>>> +		data = var->name + u16_strlen(var->name) + 1;
>>>> +
>>>> +		/*
>>>> +		 * Secure boot related and non-volatile variables shall only be
>>>> +		 * restored from U-Boot's preseed.
>>>> +		 */
>>>> +		if (!safe &&
>>>> +		    (efi_auth_var_get_type(var->name, &var->guid) !=
>>>> +		     EFI_AUTH_VAR_NONE ||
>>>> +		     !(var->attr & EFI_VARIABLE_NON_VOLATILE)))
>>>> +			continue;
>>>> +		if (!var->length)
>>>> +			continue;
>>>> +		ret = efi_var_mem_ins(var->name, &var->guid, var->attr,
>>>> +				      var->length, data, 0, NULL,
>>>> +				      var->time);
>>>> +		if (ret != EFI_SUCCESS)
>>>> +			log_err("Failed to set EFI variable %ls\n", var->name);
>>>>    	}
>>>>    	return EFI_SUCCESS;
>>>>    }
>>>> @@ -213,7 +222,7 @@ efi_status_t efi_var_from_file(void)
>>>>    		log_err("Failed to load EFI variables\n");
>>>>    		goto error;
>>>>    	}
>>>> -	if (buf->length != len || efi_var_restore(buf) != EFI_SUCCESS)
>>>> +	if (buf->length != len || efi_var_restore(buf, false) != EFI_SUCCESS)
>>>>    		log_err("Invalid EFI variables file\n");
>>>>    error:
>>>>    	free(buf);
>>>> diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c
>>>> index ba0874e9e7..a7d305ffbc 100644
>>>> --- a/lib/efi_loader/efi_variable.c
>>>> +++ b/lib/efi_loader/efi_variable.c
>>>> @@ -426,7 +426,7 @@ efi_status_t efi_init_variables(void)
>>>>
>>>>    	if (IS_ENABLED(CONFIG_EFI_VARIABLES_PRESEED)) {
>>>>    		ret = efi_var_restore((struct efi_var_file *)
>>>> -				      __efi_var_file_begin);
>>>> +				      __efi_var_file_begin, true);
>>>>    		if (ret != EFI_SUCCESS)
>>>>    			log_err("Invalid EFI variable seed\n");
>>>>    	}
>>>> --
>>>> 2.30.2
>>>>
>>