[PATCH] udf: limit the maximum number of allocation extents

[Vegard Nossum <vegard.nossum@oracle.com>]

Hit this kernel hang too while fuzzing. Please see this as a tentative
patch indicating where the problem is -- I don't really know much about
UDF or what an allocation extent is or whether there are more problems
in the same neighbourhood. It looks like udf_truncate_extents() might
also have a similar problem?

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: stable@vger.kernel.org
Cc: Jan Kara <jack@suse.com>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 fs/udf/inode.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git fs/udf/inode.c fs/udf/inode.c
index 8d0b3ad..e1875f5 100644
--- fs/udf/inode.c
+++ fs/udf/inode.c
@@ -2047,13 +2047,26 @@ void udf_write_aext(struct inode *inode, struct extent_position *epos,
 		epos->offset += adsize;
 }
 
+/*
+ * Maximum number of allocation extents. The chosen number is
+ * arbitrary - just that we hopefully don't limit any real use
+ * but avoid looping for too long on corrupted media.
+ */
+#define UDF_MAX_AEXT_NESTING 4096
+
 int8_t udf_next_aext(struct inode *inode, struct extent_position *epos,
 		     struct kernel_lb_addr *eloc, uint32_t *elen, int inc)
 {
 	int8_t etype;
+	unsigned int indirections = 0;
 
 	while ((etype = udf_current_aext(inode, epos, eloc, elen, inc)) ==
 	       (EXT_NEXT_EXTENT_ALLOCDECS >> 30)) {
+		if (++indirections > UDF_MAX_AEXT_NESTING) {
+			udf_err(inode->i_sb, "too many AEXTs (max %u supported)\n", UDF_MAX_AEXT_NESTING);
+			return -1;
+		}
+
 		int block;
 		epos->block = *eloc;
 		epos->offset = sizeof(struct allocExtDesc);
-- 
1.9.1