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Date: Thu, 4 Apr 2024 09:11:22 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Amir Goldstein <amir73il@gmail.com>
Cc: syzbot <syzbot+9a5b0ced8b1bfb238b56@syzkaller.appspotmail.com>,
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	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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On Thu, Apr 04, 2024 at 09:54:35AM +0300, Amir Goldstein wrote:
> 
> In the lockdep dependency chain, overlayfs inode lock is taken
> before kernfs internal of->mutex, where kernfs (sysfs) is the lower
> layer of overlayfs, which is sane.
> 
> With /sys/power/resume (and probably other files), sysfs also
> behaves as a stacking filesystem, calling vfs helpers, such as
> lookup_bdev() -> kern_path(), which is a behavior of a stacked
> filesystem, without all the precautions that comes with behaving
> as a stacked filesystem.

No.  This is far worse than anything stacked filesystems do - it's
an arbitrary pathname resolution while holding a lock.
It's not local.  Just about anything (including automounts, etc.)
can be happening there and it pushes the lock in question outside
of *ALL* pathwalk-related locks.  Pathname doesn't have to
resolve to anything on overlayfs - it can just go through
a symlink on it, or walk into it and traverse a bunch of ..
afterwards, etc.

Don't confuse that with stacking - it's not even close.
You can't use that anywhere near overlayfs layers.

Maybe isolate it into a separate filesystem, to be automounted
on /sys/power.  And make anyone playing with overlayfs with
sysfs as a layer mount the damn thing on top of power/ in your
overlayfs.  But using that thing as a part of layer is
a non-starter.