Re: [PATCH] rtc: Fix UBSAN overflow warning

["zhangxiaoxu (A)" <zhangxiaoxu5@huawei.com>]

On 12/20/2018 7:07 PM, Alexandre Belloni wrote:
> Hi,
> 
> On 20/12/2018 17:36:56+0800, ZhangXiaoxu wrote:
>> Users may call 'ioctl' and pass a very big value on 'tm->tm_year'.
>> It can be overflowed in 'int' after add 1900.
>> In function 'rtc_month_days' and 'mktime64', also treated it as an
>> 'unsigned' parameter.
>>
>> UBSAN: Undefined behaviour in drivers/rtc/rtc-lib.c:103:59
>> signed integer overflow:
>> 2147483647 + 1900 cannot be represented in type 'int'
>>
>> UBSAN: Undefined behaviour in drivers/rtc/rtc-lib.c:119:30
>> signed integer overflow:
>> 2147483647 + 1900 cannot be represented in type 'int'
>>
>> So, covert it to 'unsigned' explicitly.
>>
>> Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
>> ---
>>   drivers/rtc/rtc-lib.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
>> index ef160da..9714cb3 100644
>> --- a/drivers/rtc/rtc-lib.c
>> +++ b/drivers/rtc/rtc-lib.c
>> @@ -100,7 +100,7 @@ int rtc_valid_tm(struct rtc_time *tm)
>>   	if (tm->tm_year < 70
>>   		|| ((unsigned)tm->tm_mon) >= 12
>>   		|| tm->tm_mday < 1
>> -		|| tm->tm_mday > rtc_month_days(tm->tm_mon, (unsigned)(tm->tm_year + 1900))
>> +		|| tm->tm_mday > rtc_month_days(tm->tm_mon, ((unsigned)tm->tm_year + 1900))
> 
> Isn't the cast to unsigned done by rtc_month_days enough?
The undefined behaviour is 'tm->tm_year + 1900', rtc_month_days just convert the result to unsigned.
Also, signed integer overflow is undefined according to the C standard.
> 
>>   		|| ((unsigned)tm->tm_hour) >= 24
>>   		|| ((unsigned)tm->tm_min) >= 60
>>   		|| ((unsigned)tm->tm_sec) >= 60)
>> @@ -116,8 +116,8 @@ EXPORT_SYMBOL(rtc_valid_tm);
>>    */
>>   time64_t rtc_tm_to_time64(struct rtc_time *tm)
>>   {
>> -	return mktime64(tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday,
>> -			tm->tm_hour, tm->tm_min, tm->tm_sec);
>> +	return mktime64(((unsigned)tm->tm_year + 1900), tm->tm_mon + 1,
>> +			tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec);
> 
> mktime64 will fail way before tm->tm_year + 1900 overflows an int and
> also it already takes an unsigned int for year so I'm not sure this cast
> is actually necessary.
> 
>>   }
>>   EXPORT_SYMBOL(rtc_tm_to_time64);
>>   
>> -- 
>> 2.7.4
>>
>