Re: [PATCH v7 2/8] powerpc: add support to initialize ima policy rules

[Nayna <nayna@linux.vnet.ibm.com>]

On 10/15/2019 07:29 AM, Michael Ellerman wrote:
> Nayna Jain <nayna@linux.ibm.com> writes:
>> PowerNV systems uses kernel based bootloader, thus its secure boot
>> implementation uses kernel IMA security subsystem to verify the kernel
>> before kexec. Since the verification policy might differ based on the
>> secure boot mode of the system, the policies are defined at runtime.
>>
>> This patch implements the arch-specific support to define the IMA policy
>> rules based on the runtime secure boot mode of the system.
>>
>> This patch provides arch-specific IMA policies if PPC_SECURE_BOOT
>> config is enabled.
> ...
>> diff --git a/arch/powerpc/kernel/ima_arch.c b/arch/powerpc/kernel/ima_arch.c
>> new file mode 100644
>> index 000000000000..c22d82965eb4
>> --- /dev/null
>> +++ b/arch/powerpc/kernel/ima_arch.c
>> @@ -0,0 +1,33 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2019 IBM Corporation
>> + * Author: Nayna Jain
>> + */
>> +
>> +#include <linux/ima.h>
>> +#include <asm/secure_boot.h>
>> +
>> +bool arch_ima_get_secureboot(void)
>> +{
>> +	return is_powerpc_os_secureboot_enabled();
>> +}
>> +
>> +/* Defines IMA appraise rules for secureboot */
>> +static const char *const arch_rules[] = {
>> +	"appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig",
>> +#if !IS_ENABLED(CONFIG_MODULE_SIG_FORCE)
>> +	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
>> +#endif
> This confuses me.
>
> If I spell it out we get:
>
> #if IS_ENABLED(CONFIG_MODULE_SIG_FORCE)
> 	// nothing
> #else
> 	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
> #endif
>
> Which is just:
>
> #ifdef CONFIG_MODULE_SIG_FORCE
> 	// nothing
> #else
> 	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
> #endif
>
> But CONFIG_MODULE_SIG_FORCE enabled says that we *do* require modules to
> have a valid signature. Isn't that the inverse of what the rules say?
>
> Presumably I'm misunderstanding something :)

To avoid duplicate signature verification as much as possible, the IMA 
policy rule is added only if CONFIG_MODULE_SIG_FORCE is not enabled. 
CONFIG_MODULE_SIG_FORCE is part of modules support. IMA signature 
verification is based on policy.

Thanks & Regards,
      - Nayna