Re: [PATCH v5 2/2] powerpc: Add support to initialize ima policy rules

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

On 09/02/2019 07:52 AM, Michael Ellerman wrote:
> Hi Nayna,

Hi Michael,

>
> Some more comments below.
>
> Nayna Jain <nayna@linux.ibm.com> writes:
>> POWER secure boot relies on the kernel IMA security subsystem to
>> perform the OS kernel image signature verification.
> Again this is just a design choice we've made, it's not specified
> anywhere or anything like that. And it only applies to bare metal secure
> boot, at least so far. AIUI.

Yes. I will make it consistent to use "PowerNV".

>> Since each secure
>> boot mode has different IMA policy requirements, dynamic definition of
>> the policy rules based on the runtime secure boot mode of the system is
>> required. On systems that support secure boot, but have it disabled,
>> only measurement policy rules of the kernel image and modules are
>> defined.
> It's probably worth mentioning that we intend to use this in our
> Linux-based boot loader, which uses kexec, and that's one of the reasons
> why we're particularly interested in defining the rules for kexec?

Yes. Agreed. I will update patch description to add this.

>
>> This patch defines the arch-specific implementation to retrieve the
>> secure boot mode of the system and accordingly configures the IMA policy
>> rules.
>>
>> This patch provides arch-specific IMA policies if PPC_SECURE_BOOT
>> config is enabled.
>>
>> Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
>> ---
>>   arch/powerpc/Kconfig           |  2 ++
>>   arch/powerpc/kernel/Makefile   |  2 +-
>>   arch/powerpc/kernel/ima_arch.c | 50 ++++++++++++++++++++++++++++++++++
>>   include/linux/ima.h            |  3 +-
>>   4 files changed, 55 insertions(+), 2 deletions(-)
>>   create mode 100644 arch/powerpc/kernel/ima_arch.c
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index c902a39124dc..42109682b727 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -917,6 +917,8 @@ config PPC_SECURE_BOOT
>>   	bool
>>   	default n
>>   	depends on PPC64
>> +	depends on IMA
>> +	depends on IMA_ARCH_POLICY
>>   	help
>>   	  Linux on POWER with firmware secure boot enabled needs to define
>>   	  security policies to extend secure boot to the OS.This config
>> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
>> index d310ebb4e526..520b1c814197 100644
>> --- a/arch/powerpc/kernel/Makefile
>> +++ b/arch/powerpc/kernel/Makefile
>> @@ -157,7 +157,7 @@ endif
>>   obj-$(CONFIG_EPAPR_PARAVIRT)	+= epapr_paravirt.o epapr_hcalls.o
>>   obj-$(CONFIG_KVM_GUEST)		+= kvm.o kvm_emul.o
>>   
>> -obj-$(CONFIG_PPC_SECURE_BOOT)	+= secboot.o
>> +obj-$(CONFIG_PPC_SECURE_BOOT)	+= secboot.o ima_arch.o
>>   
>>   # Disable GCOV, KCOV & sanitizers in odd or sensitive code
>>   GCOV_PROFILE_prom_init.o := n
>> diff --git a/arch/powerpc/kernel/ima_arch.c b/arch/powerpc/kernel/ima_arch.c
>> new file mode 100644
>> index 000000000000..ac90fac83338
>> --- /dev/null
>> +++ b/arch/powerpc/kernel/ima_arch.c
>> @@ -0,0 +1,50 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2019 IBM Corporation
>> + * Author: Nayna Jain <nayna@linux.ibm.com>
>> + *
>> + * ima_arch.c
>> + *      - initialize ima policies for PowerPC Secure Boot
>> + */
>> +
>> +#include <linux/ima.h>
>> +#include <asm/secboot.h>
>> +
>> +bool arch_ima_get_secureboot(void)
>> +{
>> +	return get_powerpc_secureboot();
>> +}
>> +
>> +/*
>> + * File signature verification is not needed, include only measurements
>> + */
>> +static const char *const default_arch_rules[] = {
>> +	"measure func=KEXEC_KERNEL_CHECK",
>> +	"measure func=MODULE_CHECK",
>> +	NULL
>> +};
> The rules above seem fairly self explanatory.
>
>> +
>> +/* Both file signature verification and measurements are needed */
>> +static const char *const sb_arch_rules[] = {
>> +	"measure func=KEXEC_KERNEL_CHECK template=ima-modsig",
>> +	"appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig",
>> +#if IS_ENABLED(CONFIG_MODULE_SIG)
>> +	"measure func=MODULE_CHECK",
>> +#else
>> +	"measure func=MODULE_CHECK template=ima-modsig",
>> +	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
>> +#endif
> But these ones are not so obvious, at least to me who knows very little
> about IMA.
>
> Can you add a one line comment to each of the ones in here saying what
> it does and why we want it?

Sure.

>
>> +	NULL
>> +};
>> +
>> +/*
>> + * On PowerPC, file measurements are to be added to the IMA measurement list
>> + * irrespective of the secure boot state of the system.
> Why? Just because we think it's useful? Would be good to provide some
> further justification.

Sure. I will clarify this in the next version.

Thanks & Regards,
         - Nayna