linux-kernel - Re: [PATCH v3 3/3] drivers/base/core: refcount kobject and bus on device attribute read / store

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Message-ID: <YNIUZ3YZvB6rem4V@kroah.com>
Date:   Tue, 22 Jun 2021 18:48:39 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     minchan@...nel.org, jeyu@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, axboe@...nel.dk,
        mbenes@...e.com, jpoimboe@...hat.com, tglx@...utronix.de,
        keescook@...omium.org, jikos@...nel.org, rostedt@...dmis.org,
        peterz@...radead.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] drivers/base/core: refcount kobject and bus on
 device attribute read / store

On Tue, Jun 22, 2021 at 09:44:02AM -0700, Luis Chamberlain wrote:
> On Tue, Jun 22, 2021 at 09:46:26AM +0200, Greg KH wrote:
> > On Mon, Jun 21, 2021 at 04:36:51PM -0700, Luis Chamberlain wrote:
> > > It's possible today to have a device attribute read or store
> > > race against device removal. When this happens there is a small
> > > chance that the derefence for the private data area of the driver
> > > is NULL.
> > > 
> > > Let's consider the zram driver as an example. Its possible to run into
> > > a race where a sysfs knob is being used, we get preempted, and a zram
> > > device is removed before we complete use of the sysfs knob. This can happen
> > > for instance on block devices, where for instance the zram block devices
> > > just part of the private data of the block device.
> > > 
> > > For instance this can happen in the following two situations
> > > as examples to illustrate this better:
> > > 
> > >         CPU 1                            CPU 2
> > > destroy_devices
> > > ...
> > >                                  compact_store()
> > >                                  zram = dev_to_zram(dev);
> > > idr_for_each(zram_remove_cb
> > >   zram_remove
> > >   ...
> > >   kfree(zram)
> > >                                  down_read(&zram->init_lock);
> > > 
> > >         CPU 1                            CPU 2
> > > hot_remove_store
> > >                                  compact_store()
> > >                                  zram = dev_to_zram(dev);
> > >   zram_remove
> > >     kfree(zram)
> > >                                  down_read(&zram->init_lock);
> > > 
> > > To ensure the private data pointer is valid we could use bdget() / bdput()
> > > in between access, however that would mean doing that in all sysfs
> > > reads/stores on the driver. Instead a generic solution for all drivers
> > > is to ensure the device kobject is still valid and also the bus, if
> > > a bus is present.
> > > 
> > > This issue does not fix a known crash, however this race was
> > > spotted by Minchan Kim through code inspection upon code review
> > > of another zram patch.
> > > 
> > > Suggested-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> > > ---
> > >  drivers/base/base.h |  2 ++
> > >  drivers/base/bus.c  |  4 ++--
> > >  drivers/base/core.c | 42 ++++++++++++++++++++++++++++++++++++++----
> > >  3 files changed, 42 insertions(+), 6 deletions(-)
> > 
> > Please make this an independent patch of the zram mess  and I will be
> > glad to consider it for the driver core tree then.
> 
> What do you mean by making it independent?
> 
> The patch does not depend on the zram changes, and so, this can
> be merged separately as-is.

Great, then make it a 1/1 patch.  Putting it as patch 3 here means I can
not take it as our tools pull in the full series.

thanks,

greg k-h