platform/x86: dell-smbios: Add a sysfs interface for SMBIOS tokens

Currently userspace tools can access system tokens via the dcdbas
kernel module and a SMI call that will cause the platform to execute
SMM code.

With a goal in mind of deprecating the dcdbas kernel module a different
method for accessing these tokens from userspace needs to be created.

This is intentionally marked to only be readable as a process with
CAP_SYS_ADMIN as it can contain sensitive information about the
platform's configuration.

While adding this interface I found that some tokens are duplicated.
These need to be ignored from sysfs to avoid duplicate files.

MAINTAINERS was missing for this driver. Add myself and Pali to
maintainers list for it.

Signed-off-by: Mario Limonciello <[e-mail]>
Reviewed-by: Edward O'Callaghan <[e-mail]>
Signed-off-by: Darren Hart (VMware) <[e-mail]>

Documentation/ABI/testing/sysfs-platform-dell-smbios

···
+What:		/sys/devices/platform/<platform>/tokens/*
+Date:		November 2017
+KernelVersion:	4.15
+Contact:	"Mario Limonciello" <[e-mail]>
+Description:
+		A read-only description of Dell platform tokens
+		available on the machine.
+
+		Each token attribute is available as a pair of
+		sysfs attributes readable by a process with
+		CAP_SYS_ADMIN.
+
+		For example the token ID "5" would be available
+		as the following attributes:
+
+		0005_location
+		0005_value
+
+		Tokens will vary from machine to machine, and
+		only tokens available on that machine will be
+		displayed.

MAINTAINERS

···
 S:	Maintained
 F:	drivers/net/fddi/defxx.*
 
+DELL SMBIOS DRIVER
+M:	Pali Rohár <[e-mail]>
+M:	Mario Limonciello <[e-mail]>
+L:	[e-mail]
+S:	Maintained
+F:	drivers/platform/x86/dell-smbios.*
+
 DELL LAPTOP DRIVER
 M:	Matthew Garrett <[e-mail]>
 M:	Pali Rohár <[e-mail]>

drivers/platform/x86/dell-smbios.c

···
 
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/capability.h>
 #include <linux/dmi.h>
 #include <linux/err.h>
 #include <linux/gfp.h>
 #include <linux/mutex.h>
+#include <linux/platform_device.h>
 #include <linux/slab.h>
 #include <linux/io.h>
 #include "../../firmware/dcdbas.h"
···
 static int da_command_address;
 static int da_command_code;
 static int da_num_tokens;
+static struct platform_device *platform_device;
 static struct calling_interface_token *da_tokens;
+static struct device_attribute *token_location_attrs;
+static struct device_attribute *token_value_attrs;
+static struct attribute **token_attrs;
 
 int dell_smbios_error(int value)
 {
···
 	da_num_tokens += tokens;
 }
 
+static void zero_duplicates(struct device *dev)
+{
+	int i, j;
+
+	for (i = 0; i < da_num_tokens; i++) {
+		if (da_tokens[i].tokenID == 0)
+			continue;
+		for (j = i+1; j < da_num_tokens; j++) {
+			if (da_tokens[j].tokenID == 0)
+				continue;
+			if (da_tokens[i].tokenID == da_tokens[j].tokenID) {
+				dev_dbg(dev, "Zeroing dup token ID %x(%x/%x)\n",
+					da_tokens[j].tokenID,
+					da_tokens[j].location,
+					da_tokens[j].value);
+				da_tokens[j].tokenID = 0;
+			}
+		}
+	}
+}
+
 static void __init find_tokens(const struct dmi_header *dm, void *dummy)
 {
 	switch (dm->type) {
···
 		break;
 	}
 }
+
+static int match_attribute(struct device *dev,
+			   struct device_attribute *attr)
+{
+	int i;
+
+	for (i = 0; i < da_num_tokens * 2; i++) {
+		if (!token_attrs[i])
+			continue;
+		if (strcmp(token_attrs[i]->name, attr->attr.name) == 0)
+			return i/2;
+	}
+	dev_dbg(dev, "couldn't match: %s\n", attr->attr.name);
+	return -EINVAL;
+}
+
+static ssize_t location_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	int i;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	i = match_attribute(dev, attr);
+	if (i > 0)
+		return scnprintf(buf, PAGE_SIZE, "%08x", da_tokens[i].location);
+	return 0;
+}
+
+static ssize_t value_show(struct device *dev,
+			  struct device_attribute *attr, char *buf)
+{
+	int i;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	i = match_attribute(dev, attr);
+	if (i > 0)
+		return scnprintf(buf, PAGE_SIZE, "%08x", da_tokens[i].value);
+	return 0;
+}
+
+static struct attribute_group smbios_attribute_group = {
+	.name = "tokens"
+};
+
+static struct platform_driver platform_driver = {
+	.driver = {
+		.name = "dell-smbios",
+	},
+};
+
+static int build_tokens_sysfs(struct platform_device *dev)
+{
+	char buffer_location[13];
+	char buffer_value[10];
+	char *location_name;
+	char *value_name;
+	size_t size;
+	int ret;
+	int i, j;
+
+	/* (number of tokens  + 1 for null terminated */
+	size = sizeof(struct device_attribute) * (da_num_tokens + 1);
+	token_location_attrs = kzalloc(size, GFP_KERNEL);
+	if (!token_location_attrs)
+		return -ENOMEM;
+	token_value_attrs = kzalloc(size, GFP_KERNEL);
+	if (!token_value_attrs)
+		goto out_allocate_value;
+
+	/* need to store both location and value + terminator*/
+	size = sizeof(struct attribute *) * ((2 * da_num_tokens) + 1);
+	token_attrs = kzalloc(size, GFP_KERNEL);
+	if (!token_attrs)
+		goto out_allocate_attrs;
+
+	for (i = 0, j = 0; i < da_num_tokens; i++) {
+		/* skip empty */
+		if (da_tokens[i].tokenID == 0)
+			continue;
+		/* add location */
+		sprintf(buffer_location, "%04x_location",
+			da_tokens[i].tokenID);
+		location_name = kstrdup(buffer_location, GFP_KERNEL);
+		if (location_name == NULL)
+			goto out_unwind_strings;
+		sysfs_attr_init(&token_location_attrs[i].attr);
+		token_location_attrs[i].attr.name = location_name;
+		token_location_attrs[i].attr.mode = 0444;
+		token_location_attrs[i].show = location_show;
+		token_attrs[j++] = &token_location_attrs[i].attr;
+
+		/* add value */
+		sprintf(buffer_value, "%04x_value",
+			da_tokens[i].tokenID);
+		value_name = kstrdup(buffer_value, GFP_KERNEL);
+		if (value_name == NULL)
+			goto loop_fail_create_value;
+		sysfs_attr_init(&token_value_attrs[i].attr);
+		token_value_attrs[i].attr.name = value_name;
+		token_value_attrs[i].attr.mode = 0444;
+		token_value_attrs[i].show = value_show;
+		token_attrs[j++] = &token_value_attrs[i].attr;
+		continue;
+
+loop_fail_create_value:
+		kfree(value_name);
+		goto out_unwind_strings;
+	}
+	smbios_attribute_group.attrs = token_attrs;
+
+	ret = sysfs_create_group(&dev->dev.kobj, &smbios_attribute_group);
+	if (ret)
+		goto out_unwind_strings;
+	return 0;
+
+out_unwind_strings:
+	for (i = i-1; i > 0; i--) {
+		kfree(token_location_attrs[i].attr.name);
+		kfree(token_value_attrs[i].attr.name);
+	}
+	kfree(token_attrs);
+out_allocate_attrs:
+	kfree(token_value_attrs);
+out_allocate_value:
+	kfree(token_location_attrs);
+
+	return -ENOMEM;
+}
+
+static void free_group(struct platform_device *pdev)
+{
+	int i;
+
+	sysfs_remove_group(&pdev->dev.kobj,
+				&smbios_attribute_group);
+	for (i = 0; i < da_num_tokens; i++) {
+		kfree(token_location_attrs[i].attr.name);
+		kfree(token_value_attrs[i].attr.name);
+	}
+	kfree(token_attrs);
+	kfree(token_value_attrs);
+	kfree(token_location_attrs);
+}
+
 
 static int __init dell_smbios_init(void)
 {
···
 		ret = -ENOMEM;
 		goto fail_buffer;
 	}
+	ret = platform_driver_register(&platform_driver);
+	if (ret)
+		goto fail_platform_driver;
+
+	platform_device = platform_device_alloc("dell-smbios", 0);
+	if (!platform_device) {
+		ret = -ENOMEM;
+		goto fail_platform_device_alloc;
+	}
+	ret = platform_device_add(platform_device);
+	if (ret)
+		goto fail_platform_device_add;
+
+	/* duplicate tokens will cause problems building sysfs files */
+	zero_duplicates(&platform_device->dev);
+
+	ret = build_tokens_sysfs(platform_device);
+	if (ret)
+		goto fail_create_group;
 
 	return 0;
+
+fail_create_group:
+	platform_device_del(platform_device);
+
+fail_platform_device_add:
+	platform_device_put(platform_device);
+
+fail_platform_device_alloc:
+	platform_driver_unregister(&platform_driver);
+
+fail_platform_driver:
+	free_page((unsigned long)buffer);
 
 fail_buffer:
 	kfree(da_tokens);
···
 
 static void __exit dell_smbios_exit(void)
 {
-	kfree(da_tokens);
+	if (platform_device) {
+		free_group(platform_device);
+		platform_device_unregister(platform_device);
+		platform_driver_unregister(&platform_driver);
+	}
 	free_page((unsigned long)buffer);
+	kfree(da_tokens);
 }
 
 subsys_initcall(dell_smbios_init);