spdm: Allow control of next requester nonce through sysfs

Remote attestation services may mistrust the kernel to always use a
fresh nonce for SPDM authentication.

So allow user space to set the next requester nonce by writing to a
sysfs attribute.

Signed-off-by: Lukas Wunner <[email protected]>
Cc: James Bottomley <[email protected]>
Cc: Jérôme Glisse <[email protected]>
Cc: Jason Gunthorpe <[email protected]>

Author: l1k

Documentation/ABI/testing/sysfs-devices-spdm

@@ -215,3 +215,32 @@ Description:
 		necessary to parse the SPDM messages in the transcript to find
 		and extract the nonces, which is cumbersome.  That's why they
 		are exposed as separate files.
+
+
+What:		/sys/devices/.../signatures/next_requester_nonce
+Date:		June 2024
+Contact:	Lukas Wunner <[email protected]>
+Description:
+		If you do not trust the kernel to always use a fresh nonce,
+		write 32 bytes to this file to set the requester nonce used
+		in the next SPDM authentication sequence.
+
+		Meant for remote attestation services.  You are responsible
+		for providing a nonce with sufficient entropy.  The kernel
+		only uses the nonce once, so provide a new one every time
+		you reauthenticate the device.  If you do not provide a
+		nonce, the kernel generates a random one.
+
+		After the nonce has been consumed, it becomes readable as
+		the newest [0-9]*_requester_nonce, which proves its usage::
+
+		 # dd if=/dev/random bs=32 count=1 | \
+		   tee signatures/next_requester_nonce | hexdump
+		 0000000 e0 77 91 54 bd 56 99 c2 ea 4f 0b 1a 7f ba 6e 59
+		 0000010 8f ee f6 b2 26 82 58 34 9e e5 8c 8a 31 58 29 7e
+
+		 # echo re > authenticated
+
+		 # hexdump $(\ls -t signatures/[0-9]*_requester_nonce | head -1)
+		 0000000 e0 77 91 54 bd 56 99 c2 ea 4f 0b 1a 7f ba 6e 59
+		 0000010 8f ee f6 b2 26 82 58 34 9e e5 8c 8a 31 58 29 7e

lib/spdm/core.c

@@ -433,6 +433,7 @@ void spdm_destroy(struct spdm_state *spdm_state)
 	spdm_reset(spdm_state);
 	spdm_destroy_log(spdm_state);
 	mutex_destroy(&spdm_state->lock);
+	kfree(spdm_state->next_nonce);
 	kfree(spdm_state);
 }
 EXPORT_SYMBOL_GPL(spdm_destroy);

lib/spdm/req-authenticate.c

@@ -626,7 +626,13 @@ static int spdm_challenge(struct spdm_state *spdm_state, u8 slot, bool verify)
 	};
 	int rc, length;
 
-	get_random_bytes(&req.nonce, sizeof(req.nonce));
+	if (spdm_state->next_nonce) {
+		memcpy(&req.nonce, spdm_state->next_nonce, sizeof(req.nonce));
+		kfree(spdm_state->next_nonce);
+		spdm_state->next_nonce = NULL;
+	} else {
+		get_random_bytes(&req.nonce, sizeof(req.nonce));
+	}
 
 	if (spdm_state->version <= 0x12)
 		req_sz = offsetofend(typeof(req), nonce);

lib/spdm/req-sysfs.c

@@ -177,13 +177,48 @@ const struct attribute_group spdm_certificates_group = {
 
 /* signatures attributes */
 
+static umode_t spdm_signatures_are_visible(struct kobject *kobj,
+					   const struct bin_attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct spdm_state *spdm_state = dev_to_spdm_state(dev);
+
+	if (IS_ERR_OR_NULL(spdm_state))
+		return SYSFS_GROUP_INVISIBLE;
+
+	return a->attr.mode;
+}
+
+static ssize_t next_requester_nonce_write(struct file *file,
+					  struct kobject *kobj,
+					  struct bin_attribute *attr,
+					  char *buf, loff_t off, size_t count)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct spdm_state *spdm_state = dev_to_spdm_state(dev);
+
+	guard(mutex)(&spdm_state->lock);
+
+	if (!spdm_state->next_nonce) {
+		spdm_state->next_nonce = kmalloc(SPDM_NONCE_SZ, GFP_KERNEL);
+		if (!spdm_state->next_nonce)
+			return -ENOMEM;
+	}
+
+	memcpy(spdm_state->next_nonce + off, buf, count);
+	return count;
+}
+static BIN_ATTR_WO(next_requester_nonce, SPDM_NONCE_SZ);
+
 static struct bin_attribute *spdm_signatures_bin_attrs[] = {
+	&bin_attr_next_requester_nonce,
 	NULL
 };
 
 const struct attribute_group spdm_signatures_group = {
 	.name = "signatures",
 	.bin_attrs = spdm_signatures_bin_attrs,
+	.is_bin_visible = spdm_signatures_are_visible,
 };
 
 static unsigned int spdm_max_log_sz = SZ_16M; /* per device */

lib/spdm/spdm.h

@@ -475,6 +475,9 @@ struct spdm_error_rsp {
  *	itself and the transcript with trailing signature.
  * @log_counter: Number of generated log entries so far.  Will be prefixed to
  *	the sysfs files of the next generated log entry.
+ * @next_nonce: Requester nonce to be used for the next authentication
+ *	sequence.  Populated from user space through sysfs.
+ *	If user space does not provide a nonce, the kernel uses a random one.
  */
 struct spdm_state {
 	struct device *dev;
@@ -521,6 +524,7 @@ struct spdm_state {
 	struct list_head log;
 	size_t log_sz;
 	u32 log_counter;
+	u8 *next_nonce;
 };
 
 extern struct list_head spdm_state_list;