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Date: Thu, 4 Apr 2024 09:40:09 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Amir Goldstein <amir73il@gmail.com>
Cc: syzbot <syzbot+9a5b0ced8b1bfb238b56@syzkaller.appspotmail.com>,
	gregkh@linuxfoundation.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
	tj@kernel.org, valesini@yandex-team.ru,
	Christoph Hellwig <hch@lst.de>,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_fop_llseek
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Thu, Apr 04, 2024 at 09:21:10AM +0100, Al Viro wrote:

> Similar question applies to ovl_write_iter() - why do you
> need to hold the overlayfs inode locked during the call of
> backing_file_write_iter()?

Consider the scenario when unlink() is called on that sucker
during the write() that triggers that pathwalk.  We have

unlink: blocked on overlayfs inode of file, while holding the parent directory.
write: holding the overlayfs inode of file and trying to resolve a pathname
that contains .../power/suspend_stats/../../...; blocked on attempt to lock
parent so we could do a lookup in it.

No llseek involved anywhere, kernfs of->mutex held, but not contended,
deadlock purely on ->i_rwsem of overlayfs inodes.

Holding overlayfs inode locked during the call of lookup_bdev() is really
no-go.