[PATCH 5.3 146/148] staging: erofs: avoid endless loop of invalid lookback distance 0

[Greg Kroah-Hartman]

From: Gao Xiang <gaoxiang25@xxxxxxxxxx>

commit 598bb8913d015150b7734b55443c0e53e7189fc7 upstream.

As reported by erofs-utils fuzzer, Lookback distance should
be a positive number, so it should be actually looked back
rather than spinning.

Fixes: 02827e1796b3 ("staging: erofs: add erofs_map_blocks_iter")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.19+
Signed-off-by: Gao Xiang <gaoxiang25@xxxxxxxxxx>
Reviewed-by: Chao Yu <yuchao0@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20190819103426.87579-7-gaoxiang25@xxxxxxxxxx
[ Gao Xiang: Since earlier kernels don't define EFSCORRUPTED,
             let's use EIO instead. ]
Signed-off-by: Gao Xiang <gaoxiang25@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/staging/erofs/zmap.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/drivers/staging/erofs/zmap.c
+++ b/drivers/staging/erofs/zmap.c
@@ -350,6 +350,12 @@ static int vle_extent_lookback(struct z_
 
 	switch (m->type) {
 	case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
+		if (!m->delta[0]) {
+			errln("invalid lookback distance 0 at nid %llu",
+			      vi->nid);
+			DBG_BUGON(1);
+			return -EIO;
+		}
 		return vle_extent_lookback(m, m->delta[0]);
 	case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
 		map->m_flags &= ~EROFS_MAP_ZIPPED;