linux-kernel - Re: [PATCH RFC 1/2] kobject: add return value for kobject

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Date:   Tue, 18 Oct 2022 20:18:01 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@....de, axboe@...nel.dk, willy@...radead.org,
        martin.petersen@...cle.com, kch@...dia.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC 1/2] kobject: add return value for kobject_put()

On Tue, Oct 18, 2022 at 09:12:08PM +0800, Yu Kuai wrote:
> 
> 
> 在 2022/10/18 21:00, Greg KH 写道:
> > On Tue, Oct 18, 2022 at 09:14:31PM +0800, Yu Kuai wrote:
> > > The return value will be used in later patch to fix uaf for slave_dir
> > > and bd_holder_dir in block layer.
> > 
> > Then the user will be incorrect, this is not ok, you should never care
> > if you are the last "put" on an object at all.  Hint, what happens right
> > after you call this and get the result?
> > 
> 
> I tried to reset the pointer to NULL in patch 2 to prevent uaf.

That is not ok, sorry.

> And the
> whole kobject_put() and pointer reset is protected by a mutex, the mutex
> will be used on the reader side before kobject_get as well. So, in fact,
> I'm protecting them by the mutex...

Still not ok.  You never know who else has a reference on a kobject,
that's the point of reference counted objects.

> I can bypass it by using another reference anyway. But let's see if
> anyone has suggestions on the other patch.
> 
> > sorry, but NAK.
> 
> I know the best way is too refactor the lifecycle of the problematic
> bd_holder_dir/slave_dir, however, I gave that up because this seems
> quite complicated and influence is very huge...

Please fix it up properly, core changes like this should not be needed.

thanks,

greg k-h