powerpc: expose secure variables to userspace via sysfs

PowerNV secure variables, which store the keys used for OS kernel
verification, are managed by the firmware. These secure variables need to
be accessed by the userspace for addition/deletion of the certificates.

This patch adds the sysfs interface to expose secure variables for PowerNV
secureboot. The users shall use this interface for manipulating
the keys stored in the secure variables.
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Eric Richter <erichte@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1573441836-3632-3-git-send-email-nayna@linux.ibm.com

Documentation/ABI/testing/sysfs-secvar

+What:		/sys/firmware/secvar
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	This directory is created if the POWER firmware supports OS
+		secureboot, thereby secure variables. It exposes interface
+		for reading/writing the secure variables
+
+What:		/sys/firmware/secvar/vars
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	This directory lists all the secure variables that are supported
+		by the firmware.
+
+What:		/sys/firmware/secvar/format
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	A string indicating which backend is in use by the firmware.
+		This determines the format of the variable and the accepted
+		format of variable updates.
+
+What:		/sys/firmware/secvar/vars/<variable name>
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	Each secure variable is represented as a directory named as
+		<variable_name>. The variable name is unique and is in ASCII
+		representation. The data and size can be determined by reading
+		their respective attribute files.
+
+What:		/sys/firmware/secvar/vars/<variable_name>/size
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	An integer representation of the size of the content of the
+		variable. In other words, it represents the size of the data.
+
+What:		/sys/firmware/secvar/vars/<variable_name>/data
+Date:		August 2019
+Contact:	Nayna Jain h<nayna@linux.ibm.com>
+Description:	A read-only file containing the value of the variable. The size
+		of the file represents the maximum size of the variable data.
+
+What:		/sys/firmware/secvar/vars/<variable_name>/update
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	A write-only file that is used to submit the new value for the
+		variable. The size of the file represents the maximum size of
+		the variable data that can be written.

arch/powerpc/Kconfig

@@ -945,6 +945,17 @@ config PPC_SECURE_BOOT
 	  to enable OS secure boot on systems that have firmware support for
 	  it. If in doubt say N.
 
+config PPC_SECVAR_SYSFS
+	bool "Enable sysfs interface for POWER secure variables"
+	default y
+	depends on PPC_SECURE_BOOT
+	depends on SYSFS
+	help
+	  POWER secure variables are managed and controlled by firmware.
+	  These variables are exposed to userspace via sysfs to enable
+	  read/write operations on these variables. Say Y if you have
+	  secure boot enabled and want to expose variables to userspace.
+
 endmenu
 
 config ISA_DMA_API

arch/powerpc/kernel/Makefile

@@ -162,6 +162,7 @@ obj-y				+= ucall.o
 endif
 
 obj-$(CONFIG_PPC_SECURE_BOOT)	+= secure_boot.o ima_arch.o secvar-ops.o
+obj-$(CONFIG_PPC_SECVAR_SYSFS)	+= secvar-sysfs.o
 
 # Disable GCOV, KCOV & sanitizers in odd or sensitive code
 GCOV_PROFILE_prom_init.o := n

arch/powerpc/kernel/secvar-sysfs.c

+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2019 IBM Corporation <nayna@linux.ibm.com>
+ *
+ * This code exposes secure variables to user via sysfs
+ */
+
+#define pr_fmt(fmt) "secvar-sysfs: "fmt
+
+#include <linux/slab.h>
+#include <linux/compat.h>
+#include <linux/string.h>
+#include <linux/of.h>
+#include <asm/secvar.h>
+
+#define NAME_MAX_SIZE	   1024
+
+static struct kobject *secvar_kobj;
+static struct kset *secvar_kset;
+
+static ssize_t format_show(struct kobject *kobj, struct kobj_attribute *attr,
+			   char *buf)
+{
+	ssize_t rc = 0;
+	struct device_node *node;
+	const char *format;
+
+	node = of_find_compatible_node(NULL, NULL, "ibm,secvar-backend");
+	if (!of_device_is_available(node))
+		return -ENODEV;
+
+	rc = of_property_read_string(node, "format", &format);
+	if (rc)
+		return rc;
+
+	rc = sprintf(buf, "%s\n", format);
+
+	of_node_put(node);
+
+	return rc;
+}
+
+
+static ssize_t size_show(struct kobject *kobj, struct kobj_attribute *attr,
+			 char *buf)
+{
+	uint64_t dsize;
+	int rc;
+
+	rc = secvar_ops->get(kobj->name, strlen(kobj->name) + 1, NULL, &dsize);
+	if (rc) {
+		pr_err("Error retrieving %s variable size %d\n", kobj->name,
+		       rc);
+		return rc;
+	}
+
+	return sprintf(buf, "%llu\n", dsize);
+}
+
+static ssize_t data_read(struct file *filep, struct kobject *kobj,
+			 struct bin_attribute *attr, char *buf, loff_t off,
+			 size_t count)
+{
+	uint64_t dsize;
+	char *data;
+	int rc;
+
+	rc = secvar_ops->get(kobj->name, strlen(kobj->name) + 1, NULL, &dsize);
+	if (rc) {
+		pr_err("Error getting %s variable size %d\n", kobj->name, rc);
+		return rc;
+	}
+	pr_debug("dsize is %llu\n", dsize);
+
+	data = kzalloc(dsize, GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	rc = secvar_ops->get(kobj->name, strlen(kobj->name) + 1, data, &dsize);
+	if (rc) {
+		pr_err("Error getting %s variable %d\n", kobj->name, rc);
+		goto data_fail;
+	}
+
+	rc = memory_read_from_buffer(buf, count, &off, data, dsize);
+
+data_fail:
+	kfree(data);
+	return rc;
+}
+
+static ssize_t update_write(struct file *filep, struct kobject *kobj,
+			    struct bin_attribute *attr, char *buf, loff_t off,
+			    size_t count)
+{
+	int rc;
+
+	pr_debug("count is %ld\n", count);
+	rc = secvar_ops->set(kobj->name, strlen(kobj->name) + 1, buf, count);
+	if (rc) {
+		pr_err("Error setting the %s variable %d\n", kobj->name, rc);
+		return rc;
+	}
+
+	return count;
+}
+
+static struct kobj_attribute format_attr = __ATTR_RO(format);
+
+static struct kobj_attribute size_attr = __ATTR_RO(size);
+
+static struct bin_attribute data_attr = __BIN_ATTR_RO(data, 0);
+
+static struct bin_attribute update_attr = __BIN_ATTR_WO(update, 0);
+
+static struct bin_attribute *secvar_bin_attrs[] = {
+	&data_attr,
+	&update_attr,
+	NULL,
+};
+
+static struct attribute *secvar_attrs[] = {
+	&size_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group secvar_attr_group = {
+	.attrs = secvar_attrs,
+	.bin_attrs = secvar_bin_attrs,
+};
+__ATTRIBUTE_GROUPS(secvar_attr);
+
+static struct kobj_type secvar_ktype = {
+	.sysfs_ops	= &kobj_sysfs_ops,
+	.default_groups = secvar_attr_groups,
+};
+
+static int update_kobj_size(void)
+{
+
+	struct device_node *node;
+	u64 varsize;
+	int rc = 0;
+
+	node = of_find_compatible_node(NULL, NULL, "ibm,secvar-backend");
+	if (!of_device_is_available(node)) {
+		rc = -ENODEV;
+		goto out;
+	}
+
+	rc = of_property_read_u64(node, "max-var-size", &varsize);
+	if (rc)
+		goto out;
+
+	data_attr.size = varsize;
+	update_attr.size = varsize;
+
+out:
+	of_node_put(node);
+
+	return rc;
+}
+
+static int secvar_sysfs_load(void)
+{
+	char *name;
+	uint64_t namesize = 0;
+	struct kobject *kobj;
+	int rc;
+
+	name = kzalloc(NAME_MAX_SIZE, GFP_KERNEL);
+	if (!name)
+		return -ENOMEM;
+
+	do {
+		rc = secvar_ops->get_next(name, &namesize, NAME_MAX_SIZE);
+		if (rc) {
+			if (rc != -ENOENT)
+				pr_err("error getting secvar from firmware %d\n",
+				       rc);
+			break;
+		}
+
+		kobj = kzalloc(sizeof(*kobj), GFP_KERNEL);
+		if (!kobj) {
+			rc = -ENOMEM;
+			break;
+		}
+
+		kobject_init(kobj, &secvar_ktype);
+
+		rc = kobject_add(kobj, &secvar_kset->kobj, "%s", name);
+		if (rc) {
+			pr_warn("kobject_add error %d for attribute: %s\n", rc,
+				name);
+			kobject_put(kobj);
+			kobj = NULL;
+		}
+
+		if (kobj)
+			kobject_uevent(kobj, KOBJ_ADD);
+
+	} while (!rc);
+
+	kfree(name);
+	return rc;
+}
+
+static int secvar_sysfs_init(void)
+{
+	int rc;
+
+	if (!secvar_ops) {
+		pr_warn("secvar: failed to retrieve secvar operations.\n");
+		return -ENODEV;
+	}
+
+	secvar_kobj = kobject_create_and_add("secvar", firmware_kobj);
+	if (!secvar_kobj) {
+		pr_err("secvar: Failed to create firmware kobj\n");
+		return -ENOMEM;
+	}
+
+	rc = sysfs_create_file(secvar_kobj, &format_attr.attr);
+	if (rc) {
+		kobject_put(secvar_kobj);
+		return -ENOMEM;
+	}
+
+	secvar_kset = kset_create_and_add("vars", NULL, secvar_kobj);
+	if (!secvar_kset) {
+		pr_err("secvar: sysfs kobject registration failed.\n");
+		kobject_put(secvar_kobj);
+		return -ENOMEM;
+	}
+
+	rc = update_kobj_size();
+	if (rc) {
+		pr_err("Cannot read the size of the attribute\n");
+		return rc;
+	}
+
+	secvar_sysfs_load();
+
+	return 0;
+}
+
+late_initcall(secvar_sysfs_init);