Re: [PATCH v5 1/2] powerpc: detect the secure boot mode of the system

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

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

Hi Michael,

>
> Sorry I've taken so long to get to this series, there's just too many
> patches that need reviewing :/

No problem. I understand. Thanks for reviewing.

>
> Nayna Jain <nayna@linux.ibm.com> writes:
>> Secure boot on POWER defines different IMA policies based on the secure
>> boot state of the system.
> The terminology throughout is a bit vague, we have POWER, PowerPC, Linux
> on POWER etc.
>
> What this patch is talking about is a particular implemention of secure
> boot on some OpenPOWER machines running bare metal - am I right?
>
> So saying "Secure boot on POWER defines different IMA policies" is a bit
> broad I think. Really we've just decided that a way to implement secure
> boot is to use IMA policies.

I think the idea was to convey that the same design can be reused or 
extended as needed.
But I agree for now it is currently only OpenPOWER machines running on 
bare metal, I will fix the wordings to use "PowerNV" consistently.



>
>> This patch defines a function to detect the secure boot state of the
>> system.
>>
>> The PPC_SECURE_BOOT config represents the base enablement of secureboot
>> on POWER.
>>
>> Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
>> ---
>>   arch/powerpc/Kconfig               | 11 +++++
>>   arch/powerpc/include/asm/secboot.h | 27 ++++++++++++
>>   arch/powerpc/kernel/Makefile       |  2 +
>>   arch/powerpc/kernel/secboot.c      | 71 ++++++++++++++++++++++++++++++
>>   4 files changed, 111 insertions(+)
>>   create mode 100644 arch/powerpc/include/asm/secboot.h
>>   create mode 100644 arch/powerpc/kernel/secboot.c
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 77f6ebf97113..c902a39124dc 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -912,6 +912,17 @@ config PPC_MEM_KEYS
>>   
>>   	  If unsure, say y.
>>   
>> +config PPC_SECURE_BOOT
>> +	prompt "Enable PowerPC Secure Boot"
> How about "Enable secure boot support"

Yes. Sounds better.

>
>> +	bool
>> +	default n
> The default is 'n', so you don't need that default line.

Sure.


>
>> +	depends on PPC64
> Should it just depend on POWERNV for now? AFAIK there's nothing in here
> that's necessarily going to be shared with the guest secure boot code is
> there?

Yes. sounds good.


>
>> +	help
>> +	  Linux on POWER with firmware secure boot enabled needs to define
>> +	  security policies to extend secure boot to the OS.This config
>> +	  allows user to enable OS Secure Boot on PowerPC systems that
>> +	  have firmware secure boot support.
> Again POWER vs PowerPC.
>
> I think something like:
>
> "Enable support for secure boot on some systems that have firmware
> support for it. If in doubt say N."

Sure.

>
>
>> diff --git a/arch/powerpc/include/asm/secboot.h b/arch/powerpc/include/asm/secboot.h
> secure_boot.h would be fine.

Sure.

>
>> new file mode 100644
>> index 000000000000..e726261bb00b
>> --- /dev/null
>> +++ b/arch/powerpc/include/asm/secboot.h
>> @@ -0,0 +1,27 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * PowerPC secure boot definitions
>> + *
>> + * Copyright (C) 2019 IBM Corporation
>> + * Author: Nayna Jain <nayna@linux.ibm.com>
> I prefer to not have email addresses in copyright headers, as they just
> bit rot. Your email is in the git log.

Sure.


>
>> + *
>> + */
>> +#ifndef POWERPC_SECBOOT_H
>> +#define POWERPC_SECBOOT_H
> We usually do _ASM_POWERPC_SECBOOT_H (or _ASM_POWERPC_SECURE_BOOT_H).

Sure.

>
>> +#ifdef CONFIG_PPC_SECURE_BOOT
>> +extern struct device_node *is_powerpc_secvar_supported(void);
>> +extern bool get_powerpc_secureboot(void);
> You don't need 'extern' for functions in headers.

Yes. will fix.

>
>> +#else
>> +static inline struct device_node *is_powerpc_secvar_supported(void)
>> +{
>> +	return NULL;
>> +}
>> +
>> +static inline bool get_powerpc_secureboot(void)
>> +{
>> +	return false;
>> +}
>> +
>> +#endif
>> +#endif
>> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
>> index ea0c69236789..d310ebb4e526 100644
>> --- a/arch/powerpc/kernel/Makefile
>> +++ b/arch/powerpc/kernel/Makefile
>> @@ -157,6 +157,8 @@ 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
>> +
>>   # Disable GCOV, KCOV & sanitizers in odd or sensitive code
>>   GCOV_PROFILE_prom_init.o := n
>>   KCOV_INSTRUMENT_prom_init.o := n
>> diff --git a/arch/powerpc/kernel/secboot.c b/arch/powerpc/kernel/secboot.c
>> new file mode 100644
>> index 000000000000..5ea0d52d64ef
>> --- /dev/null
>> +++ b/arch/powerpc/kernel/secboot.c
>> @@ -0,0 +1,71 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2019 IBM Corporation
>> + * Author: Nayna Jain <nayna@linux.ibm.com>
>> + *
>> + * secboot.c
>> + *      - util function to get powerpc secboot state
> That's not really necessary.

Sure.

>
>> + */
>> +#include <linux/types.h>
>> +#include <linux/of.h>
>> +#include <asm/secboot.h>
>> +
>> +struct device_node *is_powerpc_secvar_supported(void)
> This is a pretty weird signature. The "is_" implies it will return a
> bool, but then it actually returns a device node *.

Yes. Agree. Will fix.

>
>> +{
>> +	struct device_node *np;
>> +	int status;
>> +
>> +	np = of_find_node_by_name(NULL, "ibm,secureboot");
>> +	if (!np) {
>> +		pr_info("secureboot node is not found\n");
>> +		return NULL;
>> +	}
> There's no good reason to search by name. You should just search by compatible.
>
> eg. of_find_compatible_node()

Sure.


>
>> +	status = of_device_is_compatible(np, "ibm,secureboot-v3");
>> +	if (!status) {
>> +		pr_info("Secure variables are not supported by this firmware\n");
>> +		return NULL;
>> +	}
>> +
>> +	return np;
>> +}
>> +
>> +bool get_powerpc_secureboot(void)
>> +{
>> +	struct device_node *np;
>> +	struct device_node *secvar_np;
>> +	const u64 *psecboot;
>> +	u64 secboot = 0;
>> +
>> +	np = is_powerpc_secvar_supported();
>> +	if (!np)
>> +		goto disabled;
>> +
>> +	/* Fail-safe for any failure related to secvar */
>> +	secvar_np = of_get_child_by_name(np, "secvar");
> Finding a child by name is not ideal, it encodes the structure of the
> tree in the API. It's better to just search by compatible.
>
> eg. of_find_compatible_node("ibm,secvar-v1")
>
> You should also define what that means, ie. write a little snippet of
> doc to define what the expected properties are and their meaning and so
> on.

It is part of the skiboot patches 
(https://patchwork.ozlabs.org/patch/1157346/)
Sure. I will add one in the kernel as well.


>
>> +	if (!secvar_np) {
>> +		pr_err("Expected secure variables support, fail-safe\n");
> I'm a bit confused by this. This is the exact opposite of what I
> understand fail-safe to mean. We shouldn't tell the user the system is
> securely booted unless we're 100% sure it is. Right?

Yes. Thanks for pointing this out. "Fail secure" is the correct term. 
This is the situation where secure variables are supported. Any failure 
reading the secure variables may be an attack, so we fail securely.


>
>> +		goto enabled;
>> +	}
>> +
>> +	if (!of_device_is_available(secvar_np)) {
>> +		pr_err("Secure variables support is in error state, fail-safe\n");
>> +		goto enabled;
>> +	}
> It seems a little weird to use the status property to indicate ok/error
> and then also have a "secure-mode" property. Wouldn't just "secure-mode"
> be sufficient with several states to represent what we need?

Before we check in which mode the system booted (e.g. setup, user, etc) 
using "secure mode" property, the "status" check helps to ensure that 
the system secure boot initialized correctly. We will look into
combining the two variables.



>
>> +	psecboot = of_get_property(secvar_np, "secure-mode", NULL);
>> +	if (!psecboot)
>> +		goto enabled;
> Please use of_read_property_u64() or similar.

Sure.

>
>> +	secboot = be64_to_cpup((__be64 *)psecboot);
>> +	if (!(secboot & (~0x0)))
> I'm not sure what that's trying to do.

We are exposing secure modes from the skiboot to the kernel via a 
bitfield, partitioned into generic modes and backend specific modes. I 
will update this code so that it is clearer.

Thanks & Regards,
       - Nayna