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

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Date:   Thu, 6 Jan 2022 09:25:22 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     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,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-fsi@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] fsi: Aspeed: Fix a potential double free

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).

> 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