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, 32 insertions, 9 deletions

diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index af683734bbdd..13b8f68f6c24 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -566,6 +566,8 @@ static int init_constants_early(struct ubifs_info *c)
 	c->ranges[UBIFS_AUTH_NODE].min_len = UBIFS_AUTH_NODE_SZ;
 	c->ranges[UBIFS_AUTH_NODE].max_len = UBIFS_AUTH_NODE_SZ +
 				UBIFS_MAX_HMAC_LEN;
+	c->ranges[UBIFS_SIG_NODE].min_len = UBIFS_SIG_NODE_SZ;
+	c->ranges[UBIFS_SIG_NODE].max_len = c->leb_size - UBIFS_SB_NODE_SZ;
 
 	c->ranges[UBIFS_INO_NODE].min_len  = UBIFS_INO_NODE_SZ;
 	c->ranges[UBIFS_INO_NODE].max_len  = UBIFS_MAX_INO_NODE_SZ;
@@ -1359,6 +1361,26 @@ static int mount_ubifs(struct ubifs_info *c)
 			goto out_lpt;
 	}
 
+	/*
+	 * Handle offline signed images: Now that the master node is
+	 * written and its validation no longer depends on the hash
+	 * in the superblock, we can update the offline signed
+	 * superblock with a HMAC version,
+	 */
+	if (ubifs_authenticated(c) && ubifs_hmac_zero(c, c->sup_node->hmac)) {
+		err = ubifs_hmac_wkm(c, c->sup_node->hmac_wkm);
+		if (err)
+			goto out_lpt;
+		c->superblock_need_write = 1;
+	}
+
+	if (!c->ro_mount && c->superblock_need_write) {
+		err = ubifs_write_sb_node(c, c->sup_node);
+		if (err)
+			goto out_lpt;
+		c->superblock_need_write = 0;
+	}
+
 	err = dbg_check_idx_size(c, c->bi.old_idx_sz);
 	if (err)
 		goto out_lpt;
@@ -1643,15 +1665,6 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 	if (err)
 		goto out;
 
-	if (c->old_leb_cnt != c->leb_cnt) {
-		struct ubifs_sb_node *sup = c->sup_node;
-
-		sup->leb_cnt = cpu_to_le32(c->leb_cnt);
-		err = ubifs_write_sb_node(c, sup);
-		if (err)
-			goto out;
-	}
-
 	if (c->need_recovery) {
 		ubifs_msg(c, "completing deferred recovery");
 		err = ubifs_write_rcvrd_mst_node(c);
@@ -1683,6 +1696,16 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 			goto out;
 	}
 
+	if (c->superblock_need_write) {
+		struct ubifs_sb_node *sup = c->sup_node;
+
+		err = ubifs_write_sb_node(c, sup);
+		if (err)
+			goto out;
+
+		c->superblock_need_write = 0;
+	}
+
 	c->ileb_buf = vmalloc(c->leb_size);
 	if (!c->ileb_buf) {
 		err = -ENOMEM;