linux-kernel - Re: [PATCH v2] powerpc/mpc5xxx: Avoid dereferencing potentially freed memory

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Date:	Fri, 16 Oct 2015 22:05:55 +0200
From:	Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	benh@...nel.crashing.org, paulus@...ba.org,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH v2] powerpc/mpc5xxx: Avoid dereferencing potentially freed
 memory

Le 16/10/2015 11:49, Michael Ellerman a écrit :
> On Fri, 2015-10-16 at 08:20 +0200, Christophe JAILLET wrote:
>> Le 15/10/2015 08:36, Michael Ellerman a écrit :
>>> On Thu, 2015-10-15 at 07:56 +0200, Christophe JAILLET wrote:
>>>> Use 'of_property_read_u32()' instead of
>>>> 'of_get_property()'+pointer
>>>> dereference in order to avoid access to potentially freed memory.
>>>>
>>>> Use 'of_get_next_parent()' to simplify the while() loop and avoid
>>>> the
>>>> need of a temp variable.
>>>>
>>>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>>>> ---
>>>> v2: Use of_property_read_u32 instead of of_get_property+pointer
>>>> dereference
>>>> *** Untested ***
>>> Thanks.
>>>
>>> Can someone with an mpc5xxx test this?
>> Hi,
>> I don't think it is an issue, but while looking at another similar
>> patch, I noticed that the proposed patch adds a call to
>> be32_to_cpup()
>> (within of_property_read_u32).
>> Apparently, powerPC is a BE architecture, so this call should be a no
>> -op.
>>
>> Just wanted to point it out, in case of.
> Hi Christoph,
>
> I'm not sure I follow.
>
> The device tree is always big endian, but of_property_read_u32() does
> the
> conversion to CPU endian for you already. That is one of the advantages
> of
> using it.
>
> cheers
>

Hi,
sorry if un-clear.

What I mean is that in the patch related 
'powerpc/sysdev/mpc5xxx_clocks.c', there was no call to 'be32_to_cpup'.
So in the proposed patch, 'of_property_read_u32' adds it.

While in the patch against 'powerpc/kernel/prom.c', 'be32_to_cpup' was 
called explicitly.
So using 'of_property_read_u32' keep the same logic.


Basically the code from 'mpc5xxx_clocks.c' and from 'prom.c' were 
written the same way. I found spurious that a call to 'be32_to_cpup' was 
done in only one case.
Maybe, it was a missing in 'mpc5xxx_clocks.c'.


I don't know if it can be an issue or not. I just find it 'strange'.


CJ


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