ovl: verify index dir matches upper dir

An index dir contains persistent hardlinks to files in upper dir.
Therefore, we must never mount an existing index dir with a differnt
upper dir.

Store the upper root dir file handle in index dir inode when index
dir is created and verify the file handle before using an existing
index dir on mount.

Add an 'is_upper' flag to the overlay file handle encoding and set it
when encoding the upper root file handle. This is not critical for index
dir verification, but it is good practice towards a standard overlayfs
file handle format for NFS export.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/copy_up.c

@@ -233,7 +233,7 @@ int ovl_set_attr(struct dentry *upperdentry, struct kstat *stat)
 	return err;
 }
 
-struct ovl_fh *ovl_encode_fh(struct dentry *lower)
+struct ovl_fh *ovl_encode_fh(struct dentry *lower, bool is_upper)
 {
 	struct ovl_fh *fh;
 	int fh_type, fh_len, dwords;
@@ -272,6 +272,14 @@ struct ovl_fh *ovl_encode_fh(struct dentry *lower)
 	fh->magic = OVL_FH_MAGIC;
 	fh->type = fh_type;
 	fh->flags = OVL_FH_FLAG_CPU_ENDIAN;
+	/*
+	 * When we will want to decode an overlay dentry from this handle
+	 * and all layers are on the same fs, if we get a disconncted real
+	 * dentry when we decode fid, the only way to tell if we should assign
+	 * it to upperdentry or to lowerstack is by checking this flag.
+	 */
+	if (is_upper)
+		fh->flags |= OVL_FH_FLAG_PATH_UPPER;
 	fh->len = fh_len;
 	fh->uuid = *uuid;
 	memcpy(fh->fid, buf, buflen);
@@ -293,7 +301,7 @@ static int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
 	 * up and a pure upper inode.
 	 */
 	if (ovl_can_decode_fh(lower->d_sb)) {
-		fh = ovl_encode_fh(lower);
+		fh = ovl_encode_fh(lower, false);
 		if (IS_ERR(fh))
 			return PTR_ERR(fh);
 	}

fs/overlayfs/namei.c

@@ -350,13 +350,13 @@ static int ovl_verify_origin_fh(struct dentry *dentry, const struct ovl_fh *fh)
  * Return 0 on match, -ESTALE on mismatch, < 0 on error.
  */
 int ovl_verify_origin(struct dentry *dentry, struct vfsmount *mnt,
-		      struct dentry *origin, bool set)
+		      struct dentry *origin, bool is_upper, bool set)
 {
 	struct inode *inode;
 	struct ovl_fh *fh;
 	int err;
 
-	fh = ovl_encode_fh(origin);
+	fh = ovl_encode_fh(origin, is_upper);
 	err = PTR_ERR(fh);
 	if (IS_ERR(fh))
 		goto fail;

fs/overlayfs/overlayfs.h

@@ -42,6 +42,8 @@ enum ovl_flag {
 /* CPU byte order required for fid decoding:  */
 #define OVL_FH_FLAG_BIG_ENDIAN	(1 << 0)
 #define OVL_FH_FLAG_ANY_ENDIAN	(1 << 1)
+/* Is the real inode encoded in fid an upper inode? */
+#define OVL_FH_FLAG_PATH_UPPER	(1 << 2)
 
 #define OVL_FH_FLAG_ALL (OVL_FH_FLAG_BIG_ENDIAN | OVL_FH_FLAG_ANY_ENDIAN)
 
@@ -233,7 +235,7 @@ static inline bool ovl_is_impuredir(struct dentry *dentry)
 
 /* namei.c */
 int ovl_verify_origin(struct dentry *dentry, struct vfsmount *mnt,
-		      struct dentry *origin, bool set);
+		      struct dentry *origin, bool is_upper, bool set);
 int ovl_path_next(int idx, struct dentry *dentry, struct path *path);
 struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags);
 bool ovl_lower_positive(struct dentry *dentry);
@@ -292,4 +294,4 @@ int ovl_copy_up(struct dentry *dentry);
 int ovl_copy_up_flags(struct dentry *dentry, int flags);
 int ovl_copy_xattr(struct dentry *old, struct dentry *new);
 int ovl_set_attr(struct dentry *upper, struct kstat *stat);
-struct ovl_fh *ovl_encode_fh(struct dentry *lower);
+struct ovl_fh *ovl_encode_fh(struct dentry *lower, bool is_upper);

fs/overlayfs/super.c

@@ -1050,7 +1050,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (!(ovl_force_readonly(ufs)) && ufs->config.index) {
 		/* Verify lower root is upper root origin */
 		err = ovl_verify_origin(upperpath.dentry, ufs->lower_mnt[0],
-					stack[0].dentry, true);
+					stack[0].dentry, false, true);
 		if (err) {
 			pr_err("overlayfs: failed to verify upper root origin\n");
 			goto out_put_lower_mnt;
@@ -1062,8 +1062,17 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		if (IS_ERR(ufs->indexdir))
 			goto out_put_lower_mnt;
 
-		if (!ufs->indexdir)
+		if (ufs->indexdir) {
+			/* Verify upper root is index dir origin */
+			err = ovl_verify_origin(ufs->indexdir, ufs->upper_mnt,
+						upperpath.dentry, true, true);
+			if (err)
+				pr_err("overlayfs: failed to verify index dir origin\n");
+		}
+		if (err || !ufs->indexdir)
 			pr_warn("overlayfs: try deleting index dir or mounting with '-o index=off' to disable inodes index.\n");
+		if (err)
+			goto out_put_indexdir;
 	}
 
 	/* Show index=off/on in /proc/mounts for any of the reasons above */