linux-kernel - Re: [PATCH v2] firmware: arm_scmi: fix divide by zero when sustained_perf

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Message-ID: <bc4cb705-e9a4-d006-66f0-7b32128b39d0@arm.com>
Date:   Thu, 6 Sep 2018 18:11:56 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Olof Johansson <olof@...om.net>
Cc:     Sudeep Holla <sudeep.holla@....com>, arm@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Ionela Voinescu <ionela.voinescu@....com>,
        Quentin Perret <quentin.perret@....com>,
        Kevin Hilman <khilman@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2] firmware: arm_scmi: fix divide by zero when
 sustained_perf_level is zero



On 06/09/18 17:59, Olof Johansson wrote:
> Hi,
> 
> On Thu, Sep 06, 2018 at 04:10:39PM +0100, Sudeep Holla wrote:
>> Firmware can provide zero as values for sustained performance level and
>> corresponding sustained frequency in kHz in order to hide the actual
>> frequencies and provide only abstract values. It may endup with divide
>> by zero scenario resulting in kernel panic.
>>
>> Let's set the multiplication factor to one if either one or both of them
>> (sustained_perf_level and sustained_freq) are set to zero.
>>
>> Fixes: a9e3fbfaa0ff ("firmware: arm_scmi: add initial support for performance protocol")
>> Reported-by: Ionela Voinescu <ionela.voinescu@....com>
>> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
>> ---
>>  drivers/firmware/arm_scmi/perf.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> Hi ARM SoC team,
>>
>> Can you pick this patch directly ?
> 
> Applied, however:
> 

Thanks.

>> diff --git a/drivers/firmware/arm_scmi/perf.c b/drivers/firmware/arm_scmi/perf.c
>> index 721e6c57beae..64342944d917 100644
>> --- a/drivers/firmware/arm_scmi/perf.c
>> +++ b/drivers/firmware/arm_scmi/perf.c
>> @@ -166,7 +166,13 @@ scmi_perf_domain_attributes_get(const struct scmi_handle *handle, u32 domain,
>>  					le32_to_cpu(attr->sustained_freq_khz);
>>  		dom_info->sustained_perf_level =
>>  					le32_to_cpu(attr->sustained_perf_level);
>> -		dom_info->mult_factor =	(dom_info->sustained_freq_khz * 1000) /
>> +		if (!dom_info->sustained_freq_khz ||
>> +		    !dom_info->sustained_perf_level)
>> +			/* CPUFreq converts to kHz, hence default 1000 */
>> +			dom_info->mult_factor =	1000;
>> +		else
>> +			dom_info->mult_factor =
>> +					(dom_info->sustained_freq_khz * 1000) /
>>  					dom_info->sustained_perf_level;
>>  		memcpy(dom_info->name, attr->name, SCMI_MAX_STR_SIZE);
> 
> I noticed you do memcpy of these name strings in a few places, and use
> it as a string. Any firmware that would return a non-terminated string
> would cause problems later on. strlcpy() might be a better approach.
> 

I seem to have assumed firmware always conforms to the definition: "Null
terminated ASCII string of up to 16 bytes in length" when I initially
wrote this.

Thanks for the finding this and the suggestion, it's always safer to
protect against firmware bugs. I will find all the occurrences and fix them.

-- 
Regards,
Sudeep