linux-ext4 - Re: kernel BUG in zero_user

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Message-ID: <l2ckvuxugdhoq3wf3s7hufwn7q3togt7tususj23te4fc75h5d@itemgw27odar>
Date: Tue, 6 May 2025 13:33:36 +0200
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>, 
	Liebes Wang <wanghaichi0403@...il.com>, 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 06-05-25 10:25:06, Zhang Yi wrote:
> On 2025/5/1 19:19, Jan Kara wrote:
> > On Wed 30-04-25 04:14:32, Matthew Wilcox wrote:
> >> 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?
> > 
> > Indeed somewhat less quirky fix could be to make ext4_max_bitmap_size()
> > return one block smaller limit. Something like:
> > 
> >         /* Compute how many blocks we can address by block tree */
> >         res += ppb;
> >         res += ppb * ppb;
> >         res += ((loff_t)ppb) * ppb * ppb;
> > +	/*
> > +	 * Hole punching assumes it can map the block past end of hole to
> > +	 * tree offsets
> > +	 */
> > +	res -= 1;
> >         /* Compute how many metadata blocks are needed */
> >         meta_blocks = 1;
> >         meta_blocks += 1 + ppb;
> > 
> > The slight caveat is that in theory there could be filesystems out there
> > with so large files and then we'd stop allowing access to such files. But I
> > guess the chances are so low that it's probably worth trying.
> > 
> 
> Hmm, I suppose this approach could pose some risks to our legacy products,
> and it makes me feel uneasy. Personally, I am more inclined toward the
> current solution, unless we decide to fix the ext4_ind_remove_space()
> directly. :)

OK. I'm just curious, are you using indirect-block based inodes and using
them upto the current s_bitmap_maxbytes size? :)

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR