linux-kernel - Re: [PATCH] rtc: s5m: Remove VLA usage

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Message-ID: <4f25592d-e805-b81e-b518-4b60867ef32a@embeddedor.com>
Date:   Fri, 9 Mar 2018 22:34:55 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Sangbeom Kim <sbkim73@...sung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-samsung-soc@...r.kernel.org, linux-rtc@...r.kernel.org,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] rtc: s5m: Remove VLA usage


Hi Krzysztof,

On 03/09/2018 07:04 AM, Krzysztof Kozlowski wrote:
> On Thu, Mar 8, 2018 at 7:03 PM, Gustavo A. R. Silva
> <garsilva@...eddedor.com> wrote:
>>
>>
>> On 03/08/2018 11:58 AM, Kees Cook wrote:
>>>
>>> On Thu, Mar 8, 2018 at 9:20 AM, Gustavo A. R. Silva
>>> <gustavo@...eddedor.com> wrote:
>>>>
>>>> In preparation to enabling -Wvla, remove VLAs and replace them
>>>> with fixed-length arrays instead.
>>>>
>>>>   From a security viewpoint, the use of Variable Length Arrays can be
>>>> a vector for stack overflow attacks. Also, in general, as the code
>>>> evolves it is easy to lose track of how big a VLA can get. Thus, we
>>>> can end up having segfaults that are hard to debug.
>>>>
>>>> Also, fixed as part of the directive to remove all VLAs from
>>>> the kernel: https://lkml.org/lkml/2018/3/7/621
>>>>
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>>> ---
>>>>    drivers/rtc/rtc-s5m.c | 15 +++++++++------
>>>>    1 file changed, 9 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/rtc/rtc-s5m.c b/drivers/rtc/rtc-s5m.c
>>>> index 6deae10..2b5f4f7 100644
>>>> --- a/drivers/rtc/rtc-s5m.c
>>>> +++ b/drivers/rtc/rtc-s5m.c
>>>> @@ -38,6 +38,9 @@
>>>>     */
>>>>    #define UDR_READ_RETRY_CNT     5
>>>>
>>>> +/* Maximum number of registers for setting time/alarm0/alarm1 */
>>>> +#define MAX_NUM_TIME_REGS      8
>>>
>>>
>>> I would adjust the various const struct s5m_rtc_reg_config's
>>> .regs_count to be represented by this new define, so the stack and the
>>> structures stay in sync. Something like:
>>>
>>> static const struct s5m_rtc_reg_config s2mps13_rtc_regs = {
>>>           .regs_count             = MAX_NUM_TIME_REGS - 1,
>>>
>>> ?
>>>
>>
>> Yep. I thought about that and decided to wait for some feedback first. But
>> yeah, I think is that'd be a good change.
> 
> Define and these assignments should be somehow connected with enum
> defining the offsets for data[] (from
> include/linux/mfd/samsung/rtc.h). Otherwise we define the same in two
> places. The enum could be itself (in separate patch) moved to the
> driver because it is meaningless for others.
>

I got it.

I'll move the enum to rtc-s5m.c and add RTC_MAX_NUM_TIME_REGS at the end 
of it. I'll send a patch series for this.

Thanks for the feedback.
--
Gustavo

> Best regards,
> Krzysztof
>