linux-kernel - Re: [PATCH] fsi: Aspeed: Fix a potential double free

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Message-ID: <edd19014-3b99-fa0b-912b-e058c14401d8@wanadoo.fr>
Date:   Thu, 6 Jan 2022 19:35:56 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Guenter Roeck <linux@...ck-us.net>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     kernel-janitors@...r.kernel.org, linux-aspeed@...ts.ozlabs.org,
        alistair@...ple.id.au, linux-kernel@...r.kernel.org,
        linux-fsi@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] fsi: Aspeed: Fix a potential double free

Le 06/01/2022 à 18:25, Guenter Roeck a écrit :
> On 1/6/22 12:14 AM, Dan Carpenter wrote:
>> On Mon, Dec 27, 2021 at 07:29:07AM +0100, Greg KH wrote:
>>> On Sun, Dec 26, 2021 at 05:56:02PM +0100, Christophe JAILLET wrote:
>>>> 'aspeed' is a devm_alloc'ed, so there is no need to free it 
>>>> explicitly or
>>>> there will be a double free().
>>>
>>> A struct device can never be devm_alloced for obvious reasons.  Perhaps
>>> that is the real problem here?
>>>
>>
>> I don't understand how "aspeed" is a struct device.
>>
> 
> -static void aspeed_master_release(struct device *dev)
> -{
> -    struct fsi_master_aspeed *aspeed =
> -        to_fsi_master_aspeed(dev_to_fsi_master(dev));
> -
> -    kfree(aspeed);
> -}
> 
> So "dev" is embedded in struct fsi_master, and struct fsi_master is 
> embedded
> in struct fsi_master_aspeed. Since "struct device" is embedded, the data
> structure embedding it must be released with the release function, as is 
> done
> here. The problem is indeed that the data structure is allocated with
> devm_kzalloc(), which as Greg points out must not be devm_ allocated
> (because its lifetime does not match the lifetime of devm_ allocated
> memory).

Thanks a lot for the detailed explanation.
Crystal clear for me now.

Do you want me to send a patch to remove the devm_ or will you?

CJ

> 
>> I've been working on understanding device managed memory recently for
>> Smatch.  It's really complicated.  There are a bunch of rules/heuristics
>> that I'm slowly creating to generate new warnings but I'm a long way
>> from understanding it well myself.
>>
> 
> A data structure embedding struct device must not be devm_ allocated,
> and it must be released with the release callback. Maybe there is
> a means to flag that somehow ?
> 
> Guenter
>