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

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Message-ID: <CAJKOXPf+4tF4oq5ePsgcpUAQS_M2Vi6b1+J-8D9CVemO-6ve5g@mail.gmail.com>
Date:   Fri, 9 Mar 2018 14:04:44 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     "Gustavo A. R. Silva" <garsilva@...eddedor.com>
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

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.

Best regards,
Krzysztof