linux-ext4 - Re: kernel BUG in zero_user

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Message-ID: <aBGVmIin8YxRyFDp@casper.infradead.org>
Date: Wed, 30 Apr 2025 04:14:32 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Zhang Yi <yi.zhang@...wei.com>
Cc: Liebes Wang <wanghaichi0403@...il.com>, Jan Kara <jack@...e.cz>,
	ojaswin@...ux.ibm.com, Theodore Ts'o <tytso@....edu>,
	linux-fsdevel@...r.kernel.org, syzkaller@...glegroups.com,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: kernel BUG in zero_user_segments

On Tue, Apr 29, 2025 at 03:55:18PM +0800, Zhang Yi wrote:
> After debugging, I found that this problem is caused by punching a hole
> with an offset variable larger than max_end on a corrupted ext4 inode,
> whose i_size is larger than maxbyte. It will result in a negative length
> in the truncate_inode_partial_folio(), which will trigger this problem.

It seems to me like we're asking for trouble when we allow an inode with
an i_size larger than max_end to be instantiated.  There are probably
other places which assume it is smaller than max_end.  We should probably
decline to create the bad inode in the first place?