misc: open-dice: Add driver to expose DICE data to userspace

Open Profile for DICE is an open protocol for measured boot compatible
with the Trusted Computing Group's Device Identifier Composition
Engine (DICE) specification. The generated Compound Device Identifier
(CDI) certificates represent the hardware/software combination measured
by DICE, and can be used for remote attestation and sealing.

Add a driver that exposes reserved memory regions populated by firmware
with DICE CDIs and exposes them to userspace via a character device.

Userspace obtains the memory region's size from read() and calls mmap()
to create a mapping of the memory region in its address space. The
mapping is not allowed to be write+shared, giving userspace a guarantee
that the data were not overwritten by another process.

Userspace can also call write(), which triggers a wipe of the DICE data
by the driver. Because both the kernel and userspace mappings use
write-combine semantics, all clients observe the memory as zeroed after
the syscall has returned.

Cc: Andrew Scull <ascull@google.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: David Brazdil <dbrazdil@google.com>

drivers/misc/Kconfig

@@ -470,6 +470,18 @@ config HISI_HIKEY_USB
 	  switching between the dual-role USB-C port and the USB-A host ports
 	  using only one USB controller.
 
+config OPEN_DICE
+	tristate "Open Profile for DICE driver"
+	depends on OF_RESERVED_MEM
+	help
+	  This driver exposes a DICE reserved memory region to userspace via
+	  a character device. The memory region contains Compound Device
+	  Identifiers (CDIs) generated by firmware as an output of DICE
+	  measured boot flow. Userspace can uses CDIs for remote attestation
+	  and sealing.
+
+	  If unsure, say N.
+
 source "drivers/misc/c2port/Kconfig"
 source "drivers/misc/eeprom/Kconfig"
 source "drivers/misc/cb710/Kconfig"

drivers/misc/Makefile

@@ -59,3 +59,4 @@ obj-$(CONFIG_UACCE)		+= uacce/
 obj-$(CONFIG_XILINX_SDFEC)	+= xilinx_sdfec.o
 obj-$(CONFIG_HISI_HIKEY_USB)	+= hisi_hikey_usb.o
 obj-$(CONFIG_HI6421V600_IRQ)	+= hi6421v600-irq.o
+obj-$(CONFIG_OPEN_DICE)		+= open-dice.o

drivers/misc/open-dice.c

@@ -0,0 +1,197 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2021 - Google LLC
+ * Author: David Brazdil <dbrazdil@google.com>
+ *
+ * Driver for Open Profile for DICE.
+ *
+ * This driver takes ownership of a reserved memory region containing secrets
+ * derived following the Open Profile for DICE. The contents of the memory
+ * region are not interpreted by the kernel but can be mapped into a userspace
+ * process via a misc device. The memory region can also be wiped, removing
+ * the secrets from memory.
+ *
+ * Userspace can access the data by (w/o error handling):
+ *
+ *     fd = open("/dev/open-dice0", O_RDWR);
+ *     size = read(fd, NULL, 0);
+ *     data = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+ *     write(fd, NULL, 0); // wipe
+ *     close(fd);
+ */
+
+#include <linux/io.h>
+#include <linux/miscdevice.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/of_reserved_mem.h>
+#include <linux/platform_device.h>
+
+#define DRIVER_NAME "open-dice"
+
+struct open_dice_drvdata {
+	spinlock_t lock;
+	char name[16];
+	struct reserved_mem *rmem;
+	struct miscdevice misc;
+};
+
+static inline struct open_dice_drvdata *to_open_dice_drvdata(struct file *filp)
+{
+	return container_of(filp->private_data, struct open_dice_drvdata, misc);
+}
+
+static int open_dice_size(struct open_dice_drvdata *drvdata)
+{
+	/* '0 < size <= INT_MAX' is checked in the probe function. */
+	return drvdata->rmem->size;
+}
+
+static int open_dice_wipe(struct open_dice_drvdata *drvdata)
+{
+	void *kaddr;
+
+	spin_lock(&drvdata->lock);
+	kaddr = devm_memremap(drvdata->misc.this_device, drvdata->rmem->base,
+			      drvdata->rmem->size, MEMREMAP_WC);
+	if (IS_ERR(kaddr)) {
+		spin_unlock(&drvdata->lock);
+		return PTR_ERR(kaddr);
+	}
+
+	memset(kaddr, 0, drvdata->rmem->size);
+	devm_memunmap(drvdata->misc.this_device, kaddr);
+	spin_unlock(&drvdata->lock);
+	return 0;
+}
+
+/**
+ * Returns the size of the reserved memory region. The user-provided pointer is
+ * never dereferenced.
+ */
+static ssize_t open_dice_read(struct file *filp, char __user *ptr, size_t len,
+			      loff_t *off)
+{
+	return open_dice_size(to_open_dice_drvdata(filp));
+}
+
+/**
+ * Triggers a wipe of the reserved memory region. The user-provided pointer is
+ * never dereferenced.
+ */
+static ssize_t open_dice_write(struct file *filp, const char __user *ptr,
+			       size_t len, loff_t *off)
+{
+	return open_dice_wipe(to_open_dice_drvdata(filp));
+}
+
+/**
+ * Creates a mapping of the reserved memory region in a user address space.
+ */
+static int open_dice_mmap(struct file *filp, struct vm_area_struct *vma)
+{
+	struct open_dice_drvdata *drvdata = to_open_dice_drvdata(filp);
+
+	/* Do not allow userspace to modify the underlying data. */
+	if ((vma->vm_flags & VM_WRITE) && (vma->vm_flags & VM_SHARED))
+		return -EPERM;
+
+	/* Create write-combine mapping so all clients observe a wipe. */
+	vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+	vma->vm_flags |= VM_DONTCOPY | VM_DONTDUMP;
+	return vm_iomap_memory(vma, drvdata->rmem->base, drvdata->rmem->size);
+}
+
+static const struct file_operations open_dice_fops = {
+	.read = open_dice_read,
+	.write = open_dice_write,
+	.mmap = open_dice_mmap,
+};
+
+static int __init open_dice_probe(struct platform_device *pdev)
+{
+	static size_t dev_idx;
+	struct device *dev = &pdev->dev;
+	struct device_node *rmem_np;
+	struct reserved_mem *rmem;
+	struct open_dice_drvdata *drvdata;
+	int ret;
+
+	rmem_np = of_parse_phandle(dev->of_node, "memory-region", 0);
+	if (!rmem_np) {
+		dev_err(dev, "missing 'memory-region' property\n");
+		return -EINVAL;
+	}
+
+	rmem = of_reserved_mem_lookup(rmem_np);
+	of_node_put(rmem_np);
+	if (!rmem) {
+		dev_err(dev, "failed to lookup reserved memory\n");
+		return -EINVAL;
+	}
+
+	if (!rmem->size || (rmem->size > INT_MAX)) {
+		dev_err(dev, "invalid memory region size\n");
+		return -EINVAL;
+	}
+
+	if (!PAGE_ALIGNED(rmem->base) || !PAGE_ALIGNED(rmem->size)) {
+		dev_err(dev, "memory region must be page-aligned\n");
+		return -EINVAL;
+	}
+
+	drvdata = devm_kmalloc(dev, sizeof(*drvdata), GFP_KERNEL);
+	if (!drvdata)
+		return -ENOMEM;
+
+	*drvdata = (struct open_dice_drvdata){
+		.lock = __SPIN_LOCK_UNLOCKED(drvdata->lock),
+		.rmem = rmem,
+		.misc = (struct miscdevice){
+			.parent	= dev,
+			.name	= drvdata->name,
+			.minor	= MISC_DYNAMIC_MINOR,
+			.fops	= &open_dice_fops,
+			.mode	= 0600,
+		},
+	};
+
+	/* Index overflow check not needed. misc_register() will fail. */
+	snprintf(drvdata->name, sizeof(drvdata->name), DRIVER_NAME"%d", dev_idx++);
+
+	ret = misc_register(&drvdata->misc);
+	if (ret) {
+		dev_err(dev, "failed to register misc device '%s': %d\n",
+			drvdata->name, ret);
+		return ret;
+	}
+
+	platform_set_drvdata(pdev, drvdata);
+	return 0;
+}
+
+static int open_dice_remove(struct platform_device *pdev)
+{
+	struct open_dice_drvdata *drvdata = platform_get_drvdata(pdev);
+
+	misc_deregister(&drvdata->misc);
+	return 0;
+}
+
+static const struct of_device_id open_dice_of_match[] = {
+	{ .compatible = "google,open-dice" },
+	{},
+};
+
+static struct platform_driver open_dice_driver = {
+	.remove = open_dice_remove,
+	.driver = {
+		.name = DRIVER_NAME,
+		.of_match_table = open_dice_of_match,
+	},
+};
+
+module_platform_driver_probe(open_dice_driver, open_dice_probe);
+
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("David Brazdil <dbrazdil@google.com>");