dm crypt: add ability to use keys from the kernel key retention service

The kernel key service is a generic way to store keys for the use of
other subsystems. Currently there is no way to use kernel keys in dm-crypt.
This patch aims to fix that. Instead of key userspace may pass a key
description with preceding ':'. So message that constructs encryption
mapping now looks like this:

  <cipher> [<key>|:<key_string>] <iv_offset> <dev_path> <start> [<#opt_params> <opt_params>]

where <key_string> is in format: <key_size>:<key_type>:<key_description>

Currently we only support two elementary key types: 'user' and 'logon'.
Keys may be loaded in dm-crypt either via <key_string> or using
classical method and pass the key in hex representation directly.

dm-crypt device initialised with a key passed in hex representation may be
replaced with key passed in key_string format and vice versa.

(Based on original work by Andrey Ryabinin)

Signed-off-by: Ondrej Kozina <okozina@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Documentation/device-mapper/dm-crypt.txt

@@ -21,13 +21,30 @@ Parameters: <cipher> <key> <iv_offset> <device path> \
     /proc/crypto contains supported crypto modes
 
 <key>
-    Key used for encryption. It is encoded as a hexadecimal number.
+    Key used for encryption. It is encoded either as a hexadecimal number
+    or it can be passed as <key_string> prefixed with single colon
+    character (':') for keys residing in kernel keyring service.
     You can only use key sizes that are valid for the selected cipher
     in combination with the selected iv mode.
     Note that for some iv modes the key string can contain additional
     keys (for example IV seed) so the key contains more parts concatenated
     into a single string.
 
+<key_string>
+    The kernel keyring key is identified by string in following format:
+    <key_size>:<key_type>:<key_description>.
+
+<key_size>
+    The encryption key size in bytes. The kernel key payload size must match
+    the value passed in <key_size>.
+
+<key_type>
+    Either 'logon' or 'user' kernel key type.
+
+<key_description>
+    The kernel keyring key description crypt target should look for
+    when loading key of <key_type>.
+
 <keycount>
     Multi-key compatibility mode. You can define <keycount> keys and
     then sectors are encrypted according to their offsets (sector 0 uses key0;
@@ -88,6 +105,12 @@ https://gitlab.com/cryptsetup/cryptsetup
 dmsetup create crypt1 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 babebabebabebabebabebabebabebabe 0 $1 0"
 ]]
 
+[[
+#!/bin/sh
+# Create a crypt device using dmsetup when encryption key is stored in keyring service
+dmsetup create crypt2 --table "0 `blockdev --getsize $1` crypt aes-cbc-essiv:sha256 :32:logon:my_prefix:my_key 0 $1 0"
+]]
+
 [[
 #!/bin/sh
 # Create a crypt device using cryptsetup and LUKS header with default cipher

drivers/md/dm-crypt.c

@@ -12,6 +12,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
+#include <linux/key.h>
 #include <linux/bio.h>
 #include <linux/blkdev.h>
 #include <linux/mempool.h>
@@ -29,6 +30,7 @@
 #include <crypto/md5.h>
 #include <crypto/algapi.h>
 #include <crypto/skcipher.h>
+#include <keys/user-type.h>
 
 #include <linux/device-mapper.h>
 
@@ -140,6 +142,7 @@ struct crypt_config {
 
 	char *cipher;
 	char *cipher_string;
+	char *key_string;
 
 	const struct crypt_iv_operations *iv_gen_ops;
 	union {
@@ -1484,22 +1487,134 @@ static int crypt_setkey(struct crypt_config *cc)
 	return err;
 }
 
+#ifdef CONFIG_KEYS
+
+static int crypt_set_keyring_key(struct crypt_config *cc, const char *key_string)
+{
+	char *new_key_string, *key_desc;
+	int ret;
+	struct key *key;
+	const struct user_key_payload *ukp;
+
+	/* look for next ':' separating key_type from key_description */
+	key_desc = strpbrk(key_string, ":");
+	if (!key_desc || key_desc == key_string || !strlen(key_desc + 1))
+		return -EINVAL;
+
+	if (strncmp(key_string, "logon:", key_desc - key_string + 1) &&
+	    strncmp(key_string, "user:", key_desc - key_string + 1))
+		return -EINVAL;
+
+	new_key_string = kstrdup(key_string, GFP_KERNEL);
+	if (!new_key_string)
+		return -ENOMEM;
+
+	key = request_key(key_string[0] == 'l' ? &key_type_logon : &key_type_user,
+			  key_desc + 1, NULL);
+	if (IS_ERR(key)) {
+		kzfree(new_key_string);
+		return PTR_ERR(key);
+	}
+
+	rcu_read_lock();
+
+	ukp = user_key_payload(key);
+	if (!ukp) {
+		rcu_read_unlock();
+		key_put(key);
+		kzfree(new_key_string);
+		return -EKEYREVOKED;
+	}
+
+	if (cc->key_size != ukp->datalen) {
+		rcu_read_unlock();
+		key_put(key);
+		kzfree(new_key_string);
+		return -EINVAL;
+	}
+
+	memcpy(cc->key, ukp->data, cc->key_size);
+
+	rcu_read_unlock();
+	key_put(key);
+
+	/* clear the flag since following operations may invalidate previously valid key */
+	clear_bit(DM_CRYPT_KEY_VALID, &cc->flags);
+
+	ret = crypt_setkey(cc);
+
+	/* wipe the kernel key payload copy in each case */
+	memset(cc->key, 0, cc->key_size * sizeof(u8));
+
+	if (!ret) {
+		set_bit(DM_CRYPT_KEY_VALID, &cc->flags);
+		kzfree(cc->key_string);
+		cc->key_string = new_key_string;
+	} else
+		kzfree(new_key_string);
+
+	return ret;
+}
+
+static int get_key_size(char **key_string)
+{
+	char *colon, dummy;
+	int ret;
+
+	if (*key_string[0] != ':')
+		return strlen(*key_string) >> 1;
+
+	/* look for next ':' in key string */
+	colon = strpbrk(*key_string + 1, ":");
+	if (!colon)
+		return -EINVAL;
+
+	if (sscanf(*key_string + 1, "%u%c", &ret, &dummy) != 2 || dummy != ':')
+		return -EINVAL;
+
+	*key_string = colon;
+
+	/* remaining key string should be :<logon|user>:<key_desc> */
+
+	return ret;
+}
+
+#else
+
+static int crypt_set_keyring_key(struct crypt_config *cc, const char *key_string)
+{
+	return -EINVAL;
+}
+
+static int get_key_size(char **key_string)
+{
+	return (*key_string[0] == ':') ? -EINVAL : strlen(*key_string) >> 1;
+}
+
+#endif
+
 static int crypt_set_key(struct crypt_config *cc, char *key)
 {
 	int r = -EINVAL;
 	int key_string_len = strlen(key);
 
-	/* The key size may not be changed. */
-	if (cc->key_size != (key_string_len >> 1))
-		goto out;
-
 	/* Hyphen (which gives a key_size of zero) means there is no key. */
 	if (!cc->key_size && strcmp(key, "-"))
 		goto out;
 
+	/* ':' means the key is in kernel keyring, short-circuit normal key processing */
+	if (key[0] == ':') {
+		r = crypt_set_keyring_key(cc, key + 1);
+		goto out;
+	}
+
 	/* clear the flag since following operations may invalidate previously valid key */
 	clear_bit(DM_CRYPT_KEY_VALID, &cc->flags);
 
+	/* wipe references to any kernel keyring key */
+	kzfree(cc->key_string);
+	cc->key_string = NULL;
+
 	if (cc->key_size && crypt_decode_key(cc->key, key, cc->key_size) < 0)
 		goto out;
 
@@ -1518,6 +1633,8 @@ static int crypt_wipe_key(struct crypt_config *cc)
 {
 	clear_bit(DM_CRYPT_KEY_VALID, &cc->flags);
 	memset(&cc->key, 0, cc->key_size * sizeof(u8));
+	kzfree(cc->key_string);
+	cc->key_string = NULL;
 
 	return crypt_setkey(cc);
 }
@@ -1555,6 +1672,7 @@ static void crypt_dtr(struct dm_target *ti)
 
 	kzfree(cc->cipher);
 	kzfree(cc->cipher_string);
+	kzfree(cc->key_string);
 
 	/* Must zero key material before freeing */
 	kzfree(cc);
@@ -1723,12 +1841,13 @@ bad_mem:
 
 /*
  * Construct an encryption mapping:
- * <cipher> <key> <iv_offset> <dev_path> <start>
+ * <cipher> [<key>|:<key_size>:<user|logon>:<key_description>] <iv_offset> <dev_path> <start>
  */
 static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 {
 	struct crypt_config *cc;
-	unsigned int key_size, opt_params;
+	int key_size;
+	unsigned int opt_params;
 	unsigned long long tmpll;
 	int ret;
 	size_t iv_size_padding;
@@ -1745,7 +1864,11 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		return -EINVAL;
 	}
 
-	key_size = strlen(argv[1]) >> 1;
+	key_size = get_key_size(&argv[1]);
+	if (key_size < 0) {
+		ti->error = "Cannot parse key size";
+		return -EINVAL;
+	}
 
 	cc = kzalloc(sizeof(*cc) + key_size * sizeof(u8), GFP_KERNEL);
 	if (!cc) {
@@ -1952,10 +2075,13 @@ static void crypt_status(struct dm_target *ti, status_type_t type,
 	case STATUSTYPE_TABLE:
 		DMEMIT("%s ", cc->cipher_string);
 
-		if (cc->key_size > 0)
-			for (i = 0; i < cc->key_size; i++)
-				DMEMIT("%02x", cc->key[i]);
-		else
+		if (cc->key_size > 0) {
+			if (cc->key_string)
+				DMEMIT(":%u:%s", cc->key_size, cc->key_string);
+			else
+				for (i = 0; i < cc->key_size; i++)
+					DMEMIT("%02x", cc->key[i]);
+		} else
 			DMEMIT("-");
 
 		DMEMIT(" %llu %s %llu", (unsigned long long)cc->iv_offset,
@@ -2011,7 +2137,7 @@ static void crypt_resume(struct dm_target *ti)
 static int crypt_message(struct dm_target *ti, unsigned argc, char **argv)
 {
 	struct crypt_config *cc = ti->private;
-	int ret = -EINVAL;
+	int key_size, ret = -EINVAL;
 
 	if (argc < 2)
 		goto error;
@@ -2022,6 +2148,13 @@ static int crypt_message(struct dm_target *ti, unsigned argc, char **argv)
 			return -EINVAL;
 		}
 		if (argc == 3 && !strcasecmp(argv[1], "set")) {
+			/* The key size may not be changed. */
+			key_size = get_key_size(&argv[2]);
+			if (key_size < 0 || cc->key_size != key_size) {
+				memset(argv[2], '0', strlen(argv[2]));
+				return -EINVAL;
+			}
+
 			ret = crypt_set_key(cc, argv[2]);
 			if (ret)
 				return ret;
@@ -2065,7 +2198,7 @@ static void crypt_io_hints(struct dm_target *ti, struct queue_limits *limits)
 
 static struct target_type crypt_target = {
 	.name   = "crypt",
-	.version = {1, 14, 1},
+	.version = {1, 15, 0},
 	.module = THIS_MODULE,
 	.ctr    = crypt_ctr,
 	.dtr    = crypt_dtr,