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

[Amir Goldstein]

On Fri, Apr 5, 2024 at 1:47 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Thu, Apr 04, 2024 at 12:33:40PM +0300, Amir Goldstein wrote:
> > On Thu, Apr 4, 2024 at 11:21 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Apr 04, 2024 at 09:11:22AM +0100, Al Viro wrote:
> > > > 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.
> >
> > I don't follow what you are saying.
> > Which code is in non-starter violation?
> > kernfs for calling lookup_bdev() with internal of->mutex held?
> > Overlayfs for allowing sysfs as a lower layer and calling
> > vfs_llseek(lower_sysfs_file,...) during copy up while ovl inode is held
> > for legit reasons (e.g. from ovl_rename())?
> >
> > >
> > > Incidentally, why do you need to lock overlayfs inode to call
> > > vfs_llseek() on the underlying file?  It might (or might not)
> > > need to lock the underlying file (for things like ->i_size,
> > > etc.), but that will be done by ->llseek() instance and it
> > > would deal with the inode in the layer, not overlayfs one.
> >
> > We do not (anymore) lock ovl inode in ovl_llseek(), see:
> > b1f9d3858f72 ovl: use ovl_inode_lock in ovl_llseek()
> > but ovl inode is held in operations (e.g. ovl_rename)
> > which trigger copy up and call vfs_llseek() on the lower file.
> >
> > >
> > > 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()?
> > >
> >
> > Not sure. This question I need to defer to Miklos.
> > I see in several places the pattern:
> >         inode_lock(inode);
> >         /* Update mode */
> >         ovl_copyattr(inode);
> >         ret = file_remove_privs(file);
> > ...
> >         /* Update size */
> >         ovl_file_modified(file);
> > ...
> >         inode_unlock(inode);
> >
> > so it could be related to atomic remove privs and update mtime,
> > but possibly we could convert all of those inode_lock() to
> > ovl_inode_lock() (i.e. internal lock below vfs inode lock).
> >
> > [...]
> > > 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.
> >
> > This specifically cannot happen because sysfs is not allowed as an
> > upper layer only as a lower layer, so overlayfs itself will not be writing to
> > /sys/power/resume.
>
> I don't understand that part. If overlayfs uses /sys/power/ as a lower
> layer it can open and write to /sys/power/resume, no?
>
> Honestly, why don't you just block /sys/power from appearing in any
> layer in overlayfs? This seems like such a niche use-case that it's so
> unlikely that this will be used that I would just try and kill it.

I do not want to special case /sys/power in overlayfs.

>
> If you do it like Al suggested and switch it to an automount you get

Not important enough IMO to make this change.

> that for free. But I guess you can also just block it without that.
>
> (Frankly, I find it weird that sysfs is allowed as a layer in any case. I
> completely forgot about this. Imho, both procfs and sysfs should not be
> usable as a lower layer - procfs is, I know - and then only select parts
> should be like /sys/fs/cgroup or sm where I can see the container people
> somehow using this to mess with the cgroup tree or something.)
>

I do not know if using sysfs as a lower layer is an important use case,
but I have a feeling that people already may do it, so I cannot regress it
without a good reason.

Al's suggestion to annotate writable kernfs files as a different class from
readonly kernfs files seems fine by me to silence lockdep false positive.

I will try to feed this solution to syzbot.

Thanks,
Amir.