firmware: turris-mox-rwtm: support ECDSA signatures via debugfs

The firmware on Turris MOX secure processor offers signing messages
with ECDSA private key stored in protected OTP memory.

The optimal solution would be to register an akcipher provider via
kernel's crypto API, but crypto API does not yet support accessing
akcipher API from userspace (and probably won't for some time, see
https://www.spinics.net/lists/linux-crypto/msg38388.html).

At first I tried to put this via standard sysfs API, but the way I
designed it is not compatible with sysfs's standard "one file per
attribute".

This patch therefore adds support for accessing this signature
generation mechanism via debugfs. Since CZ.NIC's Turris MOX is the only
user of this module, the potential future change to akcipher API should
not cause problems, since we can just change our userspace software then.

Signed-off-by: Marek Behún <[email protected]>
Signed-off-by: Gregory CLEMENT <[email protected]>

Author: elkablo

drivers/firmware/turris-mox-rwtm.c

@@ -7,6 +7,7 @@
 
 #include <linux/armada-37xx-rwtm-mailbox.h>
 #include <linux/completion.h>
+#include <linux/debugfs.h>
 #include <linux/dma-mapping.h>
 #include <linux/hw_random.h>
 #include <linux/mailbox_client.h>
@@ -69,6 +70,18 @@ struct mox_rwtm {
 	/* public key burned in eFuse */
 	int has_pubkey;
 	u8 pubkey[135];
+
+#ifdef CONFIG_DEBUG_FS
+	/*
+	 * Signature process. This is currently done via debugfs, because it
+	 * does not conform to the sysfs standard "one file per attribute".
+	 * It should be rewritten via crypto API once akcipher API is available
+	 * from userspace.
+	 */
+	struct dentry *debugfs_root;
+	u32 last_sig[34];
+	int last_sig_done;
+#endif
 };
 
 struct mox_kobject {
@@ -279,6 +292,152 @@ static int mox_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 	return ret;
 }
 
+#ifdef CONFIG_DEBUG_FS
+static int rwtm_debug_open(struct inode *inode, struct file *file)
+{
+	file->private_data = inode->i_private;
+
+	return nonseekable_open(inode, file);
+}
+
+static ssize_t do_sign_read(struct file *file, char __user *buf, size_t len,
+			    loff_t *ppos)
+{
+	struct mox_rwtm *rwtm = file->private_data;
+	ssize_t ret;
+
+	/* only allow one read, of 136 bytes, from position 0 */
+	if (*ppos != 0)
+		return 0;
+
+	if (len < 136)
+		return -EINVAL;
+
+	if (!rwtm->last_sig_done)
+		return -ENODATA;
+
+	/* 2 arrays of 17 32-bit words are 136 bytes */
+	ret = simple_read_from_buffer(buf, len, ppos, rwtm->last_sig, 136);
+	rwtm->last_sig_done = 0;
+
+	return ret;
+}
+
+static ssize_t do_sign_write(struct file *file, const char __user *buf,
+			     size_t len, loff_t *ppos)
+{
+	struct mox_rwtm *rwtm = file->private_data;
+	struct armada_37xx_rwtm_rx_msg *reply = &rwtm->reply;
+	struct armada_37xx_rwtm_tx_msg msg;
+	loff_t dummy = 0;
+	ssize_t ret;
+
+	/* the input is a SHA-512 hash, so exactly 64 bytes have to be read */
+	if (len != 64)
+		return -EINVAL;
+
+	/* if last result is not zero user has not read that information yet */
+	if (rwtm->last_sig_done)
+		return -EBUSY;
+
+	if (!mutex_trylock(&rwtm->busy))
+		return -EBUSY;
+
+	/*
+	 * Here we have to send:
+	 *   1. Address of the input to sign.
+	 *      The input is an array of 17 32-bit words, the first (most
+	 *      significat) is 0, the rest 16 words are copied from the SHA-512
+	 *      hash given by the user and converted from BE to LE.
+	 *   2. Address of the buffer where ECDSA signature value R shall be
+	 *      stored by the rWTM firmware.
+	 *   3. Address of the buffer where ECDSA signature value S shall be
+	 *      stored by the rWTM firmware.
+	 */
+	memset(rwtm->buf, 0, 4);
+	ret = simple_write_to_buffer(rwtm->buf + 4, 64, &dummy, buf, len);
+	if (ret < 0)
+		goto unlock_mutex;
+	be32_to_cpu_array(rwtm->buf, rwtm->buf, 17);
+
+	msg.command = MBOX_CMD_SIGN;
+	msg.args[0] = 1;
+	msg.args[1] = rwtm->buf_phys;
+	msg.args[2] = rwtm->buf_phys + 68;
+	msg.args[3] = rwtm->buf_phys + 2 * 68;
+	ret = mbox_send_message(rwtm->mbox, &msg);
+	if (ret < 0)
+		goto unlock_mutex;
+
+	ret = wait_for_completion_interruptible(&rwtm->cmd_done);
+	if (ret < 0)
+		goto unlock_mutex;
+
+	ret = MBOX_STS_VALUE(reply->retval);
+	if (MBOX_STS_ERROR(reply->retval) != MBOX_STS_SUCCESS)
+		goto unlock_mutex;
+
+	/*
+	 * Here we read the R and S values of the ECDSA signature
+	 * computed by the rWTM firmware and convert their words from
+	 * LE to BE.
+	 */
+	memcpy(rwtm->last_sig, rwtm->buf + 68, 136);
+	cpu_to_be32_array(rwtm->last_sig, rwtm->last_sig, 34);
+	rwtm->last_sig_done = 1;
+
+	mutex_unlock(&rwtm->busy);
+	return len;
+unlock_mutex:
+	mutex_unlock(&rwtm->busy);
+	return ret;
+}
+
+static const struct file_operations do_sign_fops = {
+	.owner	= THIS_MODULE,
+	.open	= rwtm_debug_open,
+	.read	= do_sign_read,
+	.write	= do_sign_write,
+	.llseek	= no_llseek,
+};
+
+static int rwtm_register_debugfs(struct mox_rwtm *rwtm)
+{
+	struct dentry *root, *entry;
+
+	root = debugfs_create_dir("turris-mox-rwtm", NULL);
+
+	if (IS_ERR(root))
+		return PTR_ERR(root);
+
+	entry = debugfs_create_file_unsafe("do_sign", 0600, root, rwtm,
+					   &do_sign_fops);
+	if (IS_ERR(entry))
+		goto err_remove;
+
+	rwtm->debugfs_root = root;
+
+	return 0;
+err_remove:
+	debugfs_remove_recursive(root);
+	return PTR_ERR(entry);
+}
+
+static void rwtm_unregister_debugfs(struct mox_rwtm *rwtm)
+{
+	debugfs_remove_recursive(rwtm->debugfs_root);
+}
+#else
+static inline int rwtm_register_debugfs(struct mox_rwtm *rwtm)
+{
+	return 0;
+}
+
+static inline void rwtm_unregister_debugfs(struct mox_rwtm *rwtm)
+{
+}
+#endif
+
 static int turris_mox_rwtm_probe(struct platform_device *pdev)
 {
 	struct mox_rwtm *rwtm;
@@ -340,6 +499,12 @@ static int turris_mox_rwtm_probe(struct platform_device *pdev)
 		goto free_channel;
 	}
 
+	ret = rwtm_register_debugfs(rwtm);
+	if (ret < 0) {
+		dev_err(dev, "Failed creating debugfs entries: %i\n", ret);
+		goto free_channel;
+	}
+
 	return 0;
 
 free_channel:
@@ -355,6 +520,7 @@ static int turris_mox_rwtm_remove(struct platform_device *pdev)
 {
 	struct mox_rwtm *rwtm = platform_get_drvdata(pdev);
 
+	rwtm_unregister_debugfs(rwtm);
 	sysfs_remove_files(rwtm_to_kobj(rwtm), mox_rwtm_attrs);
 	kobject_put(rwtm_to_kobj(rwtm));
 	mbox_free_channel(rwtm->mbox);