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

From: Nayna
Date: Thu Sep 05 2019 - 07:33:04 EST

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@xxxxxxxxxxxxx> 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

The PPC_SECURE_BOOT config represents the base enablement of secureboot

Signed-off-by: Nayna Jain <nayna@xxxxxxxxxxxxx>
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.
+ 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.


+ 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

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."


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


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@xxxxxxxxxxxxx>
I prefer to not have email addresses in copyright headers, as they just
bit rot. Your email is in the git log.


+ *
+ */


+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.

+static inline struct device_node *is_powerpc_secvar_supported(void)
+ return NULL;
+static inline bool get_powerpc_secureboot(void)
+ return false;
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@xxxxxxxxxxxxx>
+ *
+ * secboot.c
+ * - util function to get powerpc secboot state
That's not really necessary.


+ */
+#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()


+ 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

It is part of the skiboot patches (
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.


+ 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,
ÂÂÂÂÂ - Nayna