[PATCH v2] ext4: detect invalid INLINE_DATA + EXTENTS flag combination

[Deepanshu Kartikey <kartikey406@gmail.com>]

syzbot reported a BUG_ON in ext4_es_cache_extent() when opening a verity
file on a corrupted ext4 filesystem mounted without a journal.

The issue is that the filesystem has an inode with both the INLINE_DATA
and EXTENTS flags set:

    EXT4-fs error (device loop0): ext4_cache_extents:545: inode #15:
    comm syz.0.17: corrupted extent tree: lblk 0 < prev 66

Investigation revealed that the inode has both flags set:
    DEBUG: inode 15 - flag=1, i_inline_off=164, has_inline=1, extents_flag=1

This is an invalid combination since an inode should have either:
- INLINE_DATA: data stored directly in the inode
- EXTENTS: data stored in extent-mapped blocks

Having both flags causes ext4_has_inline_data() to return true, skipping
extent tree validation in __ext4_iget(). The unvalidated out-of-order
extents then trigger a BUG_ON in ext4_es_cache_extent() due to integer
underflow when calculating hole sizes.

Fix this by detecting this invalid flag combination early in ext4_iget()
and rejecting the corrupted inode.

Reported-and-tested-by: syzbot+038b7bf43423e132b308@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=038b7bf43423e132b308
Suggested-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
Changes in v2:
- Instead of adding validation in ext4_find_extent(), detect the invalid
  INLINE_DATA + EXTENTS flag combination in ext4_iget() as suggested by
  Zhang Yi to avoid redundant checks in the extent lookup path

 fs/ext4/inode.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5b7a15db4953..71fa3faa1475 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5445,6 +5445,15 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 	}
 
 	ret = 0;
+	/* Detect invalid flag combination - can't have both inline data and extents */
+	if (ext4_test_inode_flag(inode, EXT4_INODE_INLINE_DATA) &&
+		ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
+		ext4_error_inode(inode, __func__, __LINE__, 0,
+			"inode has both inline data and extents flags");
+		ret = -EFSCORRUPTED;
+		goto bad_inode;
+	}
+
 	if (ei->i_file_acl &&
 	    !ext4_inode_block_valid(inode, ei->i_file_acl, 1)) {
 		ext4_error_inode(inode, function, line, 0,
-- 
2.43.0