Re: [syzbot] [kernfs?] possible deadlock in kernfs_fop_llseek

[Al Viro]

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.