ubifs: support offline signed images

HMACs can only be generated on the system the UBIFS image is running on.
To support offline signed images we add a PKCS#7 signature to the UBIFS
image which can be created by mkfs.ubifs.

Both the master node and the superblock need to be authenticated, during
normal runtime both are protected with HMACs. For offline signature
support however only a single signature is desired. We add a signature
covering the superblock node directly behind it. To protect the master
node a hash of the master node is added to the superblock which is used
when the master node doesn't contain a HMAC.

Transition to a read/write filesystem is also supported. During
transition first the master node is rewritten with a HMAC (implicitly,
it is written anyway as the FS is marked dirty). Afterwards the
superblock is rewritten with a HMAC. Once after the image has been
mounted read/write it is HMAC only, the signature is no longer required
or even present on the filesystem.

In an offline signed image the master node is authenticated by the
superblock. In a transition to r/w we have to make sure that the master
node is rewritten before the superblock node. In this case the master
node gets a HMAC and its authenticity no longer depends on the
superblock node. There are some cases in which the current code first
writes the superblock node though, so with this patch writing of the
superblock node is delayed until the master node is written.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

1 files changed, 27 insertions, 1 deletions

diff --git a/fs/ubifs/ubifs-media.h b/fs/ubifs/ubifs-media.h
index 86f0f2be116c..355d3081d882 100644
--- a/fs/ubifs/ubifs-media.h
+++ b/fs/ubifs/ubifs-media.h
@@ -275,6 +275,8 @@ enum {
 #define UBIFS_CS_NODE_SZ   sizeof(struct ubifs_cs_node)
 #define UBIFS_ORPH_NODE_SZ sizeof(struct ubifs_orph_node)
 #define UBIFS_AUTH_NODE_SZ sizeof(struct ubifs_auth_node)
+#define UBIFS_SIG_NODE_SZ  sizeof(struct ubifs_sig_node)
+
 /* Extended attribute entry nodes are identical to directory entry nodes */
 #define UBIFS_XENT_NODE_SZ UBIFS_DENT_NODE_SZ
 /* Only this does not have to be multiple of 8 bytes */
@@ -301,6 +303,8 @@ enum {
  */
 #define UBIFS_XATTR_NAME_ENCRYPTION_CONTEXT "c"
 
+/* Type field in ubifs_sig_node */
+#define UBIFS_SIGNATURE_TYPE_PKCS7	1
 
 /*
  * On-flash inode flags.
@@ -361,6 +365,7 @@ enum {
  * UBIFS_CS_NODE: commit start node
  * UBIFS_ORPH_NODE: orphan node
  * UBIFS_AUTH_NODE: authentication node
+ * UBIFS_SIG_NODE: signature node
  * UBIFS_NODE_TYPES_CNT: count of supported node types
  *
  * Note, we index arrays by these numbers, so keep them low and contiguous.
@@ -381,6 +386,7 @@ enum {
 	UBIFS_CS_NODE,
 	UBIFS_ORPH_NODE,
 	UBIFS_AUTH_NODE,
+	UBIFS_SIG_NODE,
 	UBIFS_NODE_TYPES_CNT,
 };
 
@@ -638,6 +644,8 @@ struct ubifs_pad_node {
  * @hmac_wkm: HMAC of a well known message (the string "UBIFS") as a convenience
  *            to the user to check if the correct key is passed.
  * @hash_algo: The hash algo used for this filesystem (one of enum hash_algo)
+ * @hash_mst: hash of the master node, only valid for signed images in which the
+ *            master node does not contain a hmac
  */
 struct ubifs_sb_node {
 	struct ubifs_ch ch;
@@ -668,7 +676,8 @@ struct ubifs_sb_node {
 	__u8 hmac[UBIFS_MAX_HMAC_LEN];
 	__u8 hmac_wkm[UBIFS_MAX_HMAC_LEN];
 	__le16 hash_algo;
-	__u8 padding2[3838];
+	__u8 hash_mst[UBIFS_MAX_HASH_LEN];
+	__u8 padding2[3774];
 } __packed;
 
 /**
@@ -771,6 +780,23 @@ struct ubifs_auth_node {
 } __packed;
 
 /**
+ * struct ubifs_sig_node - node for signing other nodes
+ * @ch: common header
+ * @type: type of the signature, currently only UBIFS_SIGNATURE_TYPE_PKCS7
+ * supported
+ * @len: The length of the signature data
+ * @padding: reserved for future, zeroes
+ * @sig: The signature data
+ */
+struct ubifs_sig_node {
+	struct ubifs_ch ch;
+	__le32 type;
+	__le32 len;
+	__u8 padding[32];
+	__u8 sig[];
+} __packed;
+
+/**
  * struct ubifs_branch - key/reference/length branch
  * @lnum: LEB number of the target node
  * @offs: offset within @lnum