linux-kernel - Re: [PATCH 0/8] counter: Remove struct counter

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Date:   Tue, 21 Dec 2021 13:04:50 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Alexandre Torgue <alexandre.torgue@...s.st.com>,
        David Lechner <david@...hnology.com>,
        Fabrice Gasnier <fabrice.gasnier@...s.st.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Kamel Bouhara <kamel.bouhara@...tlin.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Patrick Havelange <patrick.havelange@...ensium.com>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 0/8] counter: Remove struct counter_device::priv

On 12/21/21 12:35 PM, Uwe Kleine-König wrote:
> Hello Lars,
>
> On Tue, Dec 21, 2021 at 12:12:12PM +0100, Lars-Peter Clausen wrote:
>> On 12/21/21 11:45 AM, Uwe Kleine-König wrote:
>>> similar to patch
>>> https://lore.kernel.org/r/4bde7cbd9e43a5909208102094444219d3154466.1640072891.git.vilhelm.gray@gmail.com
>>> the usage of struct counter_device::priv can be replaced by
>>> container_of which improves type safety and code size.
>>>
>>> This series depends on above patch, converts the remaining drivers and
>>> finally drops struct counter_device::priv.
>> Not sure if this is such a good idea. struct counter_device should not be
>> embedded in the drivers state struct in the first place.
> Just to mention it: My patch series didn't change this, this was already
> broken before.
I know, but this series has to be reverted when the framework is fixed.
>
>> struct counter_device contains a struct device, which is a reference counted
>> object. But by embedding it in the driver state struct the life time of both
>> the struct counter_device and and struct device are bound to the life time
>> of the driver state struct.
>>
>> Which means the struct device memory can get freed before the last reference
>> is dropped, which leads to a use-after-free and undefined behavior.
> Well, the driver struct is allocated using devm_kzalloc for all drivers.

devm_kzalloc() doesn't make a difference. The managed memory is freed 
when the parent device is unbound/removed. There may very well be 
reference to the counter_device at this point.

> So I think it's not *very* urgent to fix. Still you're right, this
> should be addressed.

Yes and no, this can trivially be used for privilege escalation, but 
then again on systems with a counter_device probably everything runs as 
root anyway.

>   
>> The framework should be changed to rather then embedding the struct
>> counter_device in the state struct to just have a pointer to it. With the
>> struct counter_device having its own allocation that will be freed when the
>> last reference to the struct device is dropped.
> My favourite would be to implement a counter_device_alloc /
> counter_device_add approach, similar to what spi_alloc_controller and
> alloc_etherdev do. The downside is that this isn't typesafe either :-\