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

[Zhang Yi <yi.zhang@huaweicloud.com>]

On 9/29/2025 11:43 PM, Deepanshu Kartikey wrote:
> 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>

Thank you for debugging and thoroughly investigating this issue! This
patch overall looks good to me, with just a few minor suggestions.

> ---
> 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)) {
	    ^^^
	    I'd recommended to maintain format alignment.

> +		ext4_error_inode(inode, __func__, __LINE__, 0,
		                        ^^^^^^^^^^^^^^^^^^
		                        function, line,> +			"inode has both inline data and extents flags");
> +		ret = -EFSCORRUPTED;
> +		goto bad_inode;
> +	}
> +

Additionally, I would prefer to move this check earlier, immediately
after setting the flags, that is, after ext4_set_inode_flags(). What
do you think?

Thanks,
Yi.

>  	if (ei->i_file_acl &&
>  	    !ext4_inode_block_valid(inode, ei->i_file_acl, 1)) {
>  		ext4_error_inode(inode, function, line, 0,