[Devel] [PATCH RH7 3/3] ext4: fix EXT_MAX_EXTENT/INDEX to check for zeroed eh_max

[Kirill Tkhai]

From: Harshad Shirwadkar <harshadshirwadkar at gmail.com>

ms commit c36a71b4e35a

If eh->eh_max is 0, EXT_MAX_EXTENT/INDEX would evaluate to unsigned
(-1) resulting in illegal memory accesses. Although there is no
consistent repro, we see that generic/019 sometimes crashes because of
this bug.

Ran gce-xfstests smoke and verified that there were no regressions.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar at gmail.com>
Link: https://lore.kernel.org/r/20200421023959.20879-2-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Cc: stable at kernel.org
---
 fs/ext4/ext4_extents.h |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/ext4_extents.h b/fs/ext4/ext4_extents.h
index 01e72cb0ad0d..dc2bc944ecab 100644
--- a/fs/ext4/ext4_extents.h
+++ b/fs/ext4/ext4_extents.h
@@ -168,10 +168,13 @@ struct ext4_ext_path {
 	(EXT_FIRST_EXTENT((__hdr__)) + le16_to_cpu((__hdr__)->eh_entries) - 1)
 #define EXT_LAST_INDEX(__hdr__) \
 	(EXT_FIRST_INDEX((__hdr__)) + le16_to_cpu((__hdr__)->eh_entries) - 1)
-#define EXT_MAX_EXTENT(__hdr__) \
-	(EXT_FIRST_EXTENT((__hdr__)) + le16_to_cpu((__hdr__)->eh_max) - 1)
+#define EXT_MAX_EXTENT(__hdr__)	\
+	((le16_to_cpu((__hdr__)->eh_max)) ? \
+	((EXT_FIRST_EXTENT((__hdr__)) + le16_to_cpu((__hdr__)->eh_max) - 1)) \
+					: 0)
 #define EXT_MAX_INDEX(__hdr__) \
-	(EXT_FIRST_INDEX((__hdr__)) + le16_to_cpu((__hdr__)->eh_max) - 1)
+	((le16_to_cpu((__hdr__)->eh_max)) ? \
+	((EXT_FIRST_INDEX((__hdr__)) + le16_to_cpu((__hdr__)->eh_max) - 1)) : 0)
 
 static inline struct ext4_extent_header *ext_inode_hdr(struct inode *inode)
 {