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

[Greg KH]

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@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
>  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.

thanks,

greg k-h