Re: [PATCH V7] squashfs: Add symlink size check in squash_read_inode

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Aug 06, 2024 at 02:19:12PM +0800, Lizhi Xu wrote:

> > However, you seem to find some problem in the latter form, and
> > your explanations of the reasons have been hard to understand.
> Here are the uninit-value related calltrace reports from syzbot:
> 
> page_get_link()->
>   read_mapping_page()->
>     read_cache_page()->
>       do_read_cache_page()->
>         do_read_cache_folio()->
>           filemap_read_folio()->
>             squashfs_symlink_read_folio()
> 
> fs/squashfs/symlink.c
>  8 static int squashfs_symlink_read_folio(struct file *file, struct folio *folio)
>  7 {
>  6         struct inode *inode = folio->mapping->host;
>  5         struct super_block *sb = inode->i_sb;
>  4         struct squashfs_sb_info *msblk = sb->s_fs_info;
>  3         int index = folio_pos(folio);
>  2         u64 block = squashfs_i(inode)->start;
>  1         int offset = squashfs_i(inode)->offset;
> 41         int length = min_t(int, i_size_read(inode) - index, PAGE_SIZE);
> 
> Please see line 41, because the value of i_size is too large, causing integer overflow
> in the variable length. Which can result in folio not being initialized (as reported by 
> Syzbot: "KMSAN: uninit-value in pick_link").

What does that have to do with anything?  In the code you've quoted, ->i_size - index
is explicitly cast to signed 32bit.  _That_ will wrap around.  On typecast.  Nothing
of that sort would be present in
	if (inode->i_size > PAGE_SIZE)
as you could have verified easily.

At that point the only thing I can recommend is googling for "first law of holes".