quota: Detect loops in quota tree

Syzbot has found that when it creates corrupted quota files where the
quota tree contains a loop, we will deadlock when tryling to insert a
dquot. Add loop detection into functions traversing the quota tree.

Signed-off-by: Jan Kara <[e-mail]>

fs/quota/quota_tree.c

···
 MODULE_DESCRIPTION("Quota trie support");
 MODULE_LICENSE("GPL");
 
+/*
+ * Maximum quota tree depth we support. Only to limit recursion when working
+ * with the tree.
+ */
+#define MAX_QTREE_DEPTH 6
+
 #define __QUOTA_QT_PARANOIA
 
 static int __get_index(struct qtree_mem_dqinfo *info, qid_t id, int depth)
···
 
 /* Insert reference to structure into the trie */
 static int do_insert_tree(struct qtree_mem_dqinfo *info, struct dquot *dquot,
-			  uint *treeblk, int depth)
+			  uint *blks, int depth)
 {
 	char *buf = kmalloc(info->dqi_usable_bs, GFP_KERNEL);
 	int ret = 0, newson = 0, newact = 0;
 	__le32 *ref;
 	uint newblk;
+	int i;
 
 	if (!buf)
 		return -ENOMEM;
-	if (!*treeblk) {
+	if (!blks[depth]) {
 		ret = get_free_dqblk(info);
 		if (ret < 0)
 			goto out_buf;
-		*treeblk = ret;
+		for (i = 0; i < depth; i++)
+			if (ret == blks[i]) {
+				quota_error(dquot->dq_sb,
+					"Free block already used in tree: block %u",
+					ret);
+				ret = -EIO;
+				goto out_buf;
+			}
+		blks[depth] = ret;
 		memset(buf, 0, info->dqi_usable_bs);
 		newact = 1;
 	} else {
-		ret = read_blk(info, *treeblk, buf);
+		ret = read_blk(info, blks[depth], buf);
 		if (ret < 0) {
 			quota_error(dquot->dq_sb, "Can't read tree quota "
-				    "block %u", *treeblk);
+				    "block %u", blks[depth]);
 			goto out_buf;
 		}
 	}
···
 			     info->dqi_blocks - 1);
 	if (ret)
 		goto out_buf;
-	if (!newblk)
+	if (!newblk) {
 		newson = 1;
+	} else {
+		for (i = 0; i <= depth; i++)
+			if (newblk == blks[i]) {
+				quota_error(dquot->dq_sb,
+					"Cycle in quota tree detected: block %u index %u",
+					blks[depth],
+					get_index(info, dquot->dq_id, depth));
+				ret = -EIO;
+				goto out_buf;
+			}
+	}
+	blks[depth + 1] = newblk;
 	if (depth == info->dqi_qtree_depth - 1) {
 #ifdef __QUOTA_QT_PARANOIA
 		if (newblk) {
···
 			goto out_buf;
 		}
 #endif
-		newblk = find_free_dqentry(info, dquot, &ret);
+		blks[depth + 1] = find_free_dqentry(info, dquot, &ret);
 	} else {
-		ret = do_insert_tree(info, dquot, &newblk, depth+1);
+		ret = do_insert_tree(info, dquot, blks, depth + 1);
 	}
 	if (newson && ret >= 0) {
 		ref[get_index(info, dquot->dq_id, depth)] =
-							cpu_to_le32(newblk);
-		ret = write_blk(info, *treeblk, buf);
+						cpu_to_le32(blks[depth + 1]);
+		ret = write_blk(info, blks[depth], buf);
 	} else if (newact && ret < 0) {
-		put_free_dqblk(info, buf, *treeblk);
+		put_free_dqblk(info, buf, blks[depth]);
 	}
 out_buf:
 	kfree(buf);
···
 static inline int dq_insert_tree(struct qtree_mem_dqinfo *info,
 				 struct dquot *dquot)
 {
-	int tmp = QT_TREEOFF;
+	uint blks[MAX_QTREE_DEPTH] = { QT_TREEOFF };
 
 #ifdef __QUOTA_QT_PARANOIA
 	if (info->dqi_blocks <= QT_TREEOFF) {
···
 		return -EIO;
 	}
 #endif
-	return do_insert_tree(info, dquot, &tmp, 0);
+	if (info->dqi_qtree_depth >= MAX_QTREE_DEPTH) {
+		quota_error(dquot->dq_sb, "Quota tree depth too big!");
+		return -EIO;
+	}
+	return do_insert_tree(info, dquot, blks, 0);
 }
 
 /*
···
 
 /* Remove reference to dquot from tree */
 static int remove_tree(struct qtree_mem_dqinfo *info, struct dquot *dquot,
-		       uint *blk, int depth)
+		       uint *blks, int depth)
 {
 	char *buf = kmalloc(info->dqi_usable_bs, GFP_KERNEL);
 	int ret = 0;
 	uint newblk;
 	__le32 *ref = (__le32 *)buf;
+	int i;
 
 	if (!buf)
 		return -ENOMEM;
-	ret = read_blk(info, *blk, buf);
+	ret = read_blk(info, blks[depth], buf);
 	if (ret < 0) {
 		quota_error(dquot->dq_sb, "Can't read quota data block %u",
-			    *blk);
+			    blks[depth]);
 		goto out_buf;
 	}
 	newblk = le32_to_cpu(ref[get_index(info, dquot->dq_id, depth)]);
···
 	if (ret)
 		goto out_buf;
 
+	for (i = 0; i <= depth; i++)
+		if (newblk == blks[i]) {
+			quota_error(dquot->dq_sb,
+				"Cycle in quota tree detected: block %u index %u",
+				blks[depth],
+				get_index(info, dquot->dq_id, depth));
+			ret = -EIO;
+			goto out_buf;
+		}
 	if (depth == info->dqi_qtree_depth - 1) {
 		ret = free_dqentry(info, dquot, newblk);
-		newblk = 0;
+		blks[depth + 1] = 0;
 	} else {
-		ret = remove_tree(info, dquot, &newblk, depth+1);
+		blks[depth + 1] = newblk;
+		ret = remove_tree(info, dquot, blks, depth + 1);
 	}
-	if (ret >= 0 && !newblk) {
-		int i;
+	if (ret >= 0 && !blks[depth + 1]) {
 		ref[get_index(info, dquot->dq_id, depth)] = cpu_to_le32(0);
 		/* Block got empty? */
 		for (i = 0; i < (info->dqi_usable_bs >> 2) && !ref[i]; i++)
 			;
 		/* Don't put the root block into the free block list */
 		if (i == (info->dqi_usable_bs >> 2)
-		    && *blk != QT_TREEOFF) {
-			put_free_dqblk(info, buf, *blk);
-			*blk = 0;
+		    && blks[depth] != QT_TREEOFF) {
+			put_free_dqblk(info, buf, blks[depth]);
+			blks[depth] = 0;
 		} else {
-			ret = write_blk(info, *blk, buf);
+			ret = write_blk(info, blks[depth], buf);
 			if (ret < 0)
 				quota_error(dquot->dq_sb,
 					    "Can't write quota tree block %u",
-					    *blk);
+					    blks[depth]);
 		}
 	}
 out_buf:
···
 /* Delete dquot from tree */
 int qtree_delete_dquot(struct qtree_mem_dqinfo *info, struct dquot *dquot)
 {
-	uint tmp = QT_TREEOFF;
+	uint blks[MAX_QTREE_DEPTH] = { QT_TREEOFF };
 
 	if (!dquot->dq_off)	/* Even not allocated? */
 		return 0;
-	return remove_tree(info, dquot, &tmp, 0);
+	if (info->dqi_qtree_depth >= MAX_QTREE_DEPTH) {
+		quota_error(dquot->dq_sb, "Quota tree depth too big!");
+		return -EIO;
+	}
+	return remove_tree(info, dquot, blks, 0);
 }
 EXPORT_SYMBOL(qtree_delete_dquot);
 
···
 
 /* Find entry for given id in the tree */
 static loff_t find_tree_dqentry(struct qtree_mem_dqinfo *info,
-				struct dquot *dquot, uint blk, int depth)
+				struct dquot *dquot, uint *blks, int depth)
 {
 	char *buf = kmalloc(info->dqi_usable_bs, GFP_KERNEL);
 	loff_t ret = 0;
 	__le32 *ref = (__le32 *)buf;
+	uint blk;
+	int i;
 
 	if (!buf)
 		return -ENOMEM;
-	ret = read_blk(info, blk, buf);
+	ret = read_blk(info, blks[depth], buf);
 	if (ret < 0) {
 		quota_error(dquot->dq_sb, "Can't read quota tree block %u",
-			    blk);
+			    blks[depth]);
 		goto out_buf;
 	}
 	ret = 0;
···
 	if (ret)
 		goto out_buf;
 
+	/* Check for cycles in the tree */
+	for (i = 0; i <= depth; i++)
+		if (blk == blks[i]) {
+			quota_error(dquot->dq_sb,
+				"Cycle in quota tree detected: block %u index %u",
+				blks[depth],
+				get_index(info, dquot->dq_id, depth));
+			ret = -EIO;
+			goto out_buf;
+		}
+	blks[depth + 1] = blk;
 	if (depth < info->dqi_qtree_depth - 1)
-		ret = find_tree_dqentry(info, dquot, blk, depth+1);
+		ret = find_tree_dqentry(info, dquot, blks, depth + 1);
 	else
 		ret = find_block_dqentry(info, dquot, blk);
 out_buf:
···
 static inline loff_t find_dqentry(struct qtree_mem_dqinfo *info,
 				  struct dquot *dquot)
 {
-	return find_tree_dqentry(info, dquot, QT_TREEOFF, 0);
+	uint blks[MAX_QTREE_DEPTH] = { QT_TREEOFF };
+
+	if (info->dqi_qtree_depth >= MAX_QTREE_DEPTH) {
+		quota_error(dquot->dq_sb, "Quota tree depth too big!");
+		return -EIO;
+	}
+	return find_tree_dqentry(info, dquot, blks, 0);
 }
 
 int qtree_read_dquot(struct qtree_mem_dqinfo *info, struct dquot *dquot)

fs/quota/quota_v2.c

···
 		    i_size_read(sb_dqopt(sb)->files[type]));
 		goto out_free;
 	}
-	if (qinfo->dqi_free_blk >= qinfo->dqi_blocks) {
-		quota_error(sb, "Free block number too big (%u >= %u).",
-			    qinfo->dqi_free_blk, qinfo->dqi_blocks);
+	if (qinfo->dqi_free_blk && (qinfo->dqi_free_blk <= QT_TREEOFF ||
+	    qinfo->dqi_free_blk >= qinfo->dqi_blocks)) {
+		quota_error(sb, "Free block number %u out of range (%u, %u).",
+			    qinfo->dqi_free_blk, QT_TREEOFF, qinfo->dqi_blocks);
 		goto out_free;
 	}
-	if (qinfo->dqi_free_entry >= qinfo->dqi_blocks) {
-		quota_error(sb, "Block with free entry too big (%u >= %u).",
-			    qinfo->dqi_free_entry, qinfo->dqi_blocks);
+	if (qinfo->dqi_free_entry && (qinfo->dqi_free_entry <= QT_TREEOFF ||
+	    qinfo->dqi_free_entry >= qinfo->dqi_blocks)) {
+		quota_error(sb, "Block with free entry %u out of range (%u, %u).",
+			    qinfo->dqi_free_entry, QT_TREEOFF,
+			    qinfo->dqi_blocks);
 		goto out_free;
 	}
 	ret = 0;