udf: limit the maximum number of TD redirections

Filesystem fuzzing revealed that we could get stuck in the
udf_process_sequence() loop.

The maximum limit was chosen arbitrarily but fixes the problem I saw.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/super.c

@@ -1585,6 +1585,13 @@ static void udf_load_logicalvolint(struct super_block *sb, struct kernel_extent_
 		brelse(bh);
 }
 
+/*
+ * Maximum number of Terminating Descriptor redirections. The chosen number is
+ * arbitrary - just that we hopefully don't limit any real use of rewritten
+ * inode on write-once media but avoid looping for too long on corrupted media.
+ */
+#define UDF_MAX_TD_NESTING 64
+
 /*
  * Process a main/reserve volume descriptor sequence.
  *   @block		First block of first extent of the sequence.
@@ -1609,6 +1616,7 @@ static noinline int udf_process_sequence(
 	uint16_t ident;
 	long next_s = 0, next_e = 0;
 	int ret;
+	unsigned int indirections = 0;
 
 	memset(vds, 0, sizeof(struct udf_vds_record) * VDS_POS_LENGTH);
 
@@ -1679,6 +1687,12 @@ static noinline int udf_process_sequence(
 			}
 			break;
 		case TAG_IDENT_TD: /* ISO 13346 3/10.9 */
+			if (++indirections > UDF_MAX_TD_NESTING) {
+				udf_err(sb, "too many TDs (max %u supported)\n", UDF_MAX_TD_NESTING);
+				brelse(bh);
+				return -EIO;
+			}
+
 			vds[VDS_POS_TERMINATING_DESC].block = block;
 			if (next_e) {
 				block = next_s;