Re: [PATCH 1/1] kernel/sysctl.c: avoid overflow

[Kees Cook]

On Tue, Jun 14, 2016 at 1:19 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, 11 Jun 2016 03:33:08 +0200 Heinrich Schuchardt <xypron.glpk@xxxxxx> wrote:
>
>> An undetected overflow may occur in do_proc_dointvec_minmax_conv_param.
>>
>> ...
>>
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -2313,7 +2313,17 @@ static int do_proc_dointvec_minmax_conv(bool *negp, unsigned long *lvalp,
>>  {
>>       struct do_proc_dointvec_minmax_conv_param *param = data;
>>       if (write) {
>> -             int val = *negp ? -*lvalp : *lvalp;
>> +             int val;
>> +
>> +             if (*negp) {
>> +                     if (*lvalp > (unsigned long) INT_MAX + 1)
>> +                             return -EINVAL;
>> +                     val = -*lvalp;
>> +             } else {
>> +                     if (*lvalp > (unsigned long) INT_MAX)
>> +                             return -EINVAL;
>> +                     val = *lvalp;
>> +             }
>>               if ((param->min && *param->min > val) ||
>>                   (param->max && *param->max < val))
>>                       return -EINVAL;
>
> hm.
>
> What happens if someone does
>
>         echo -1 > /proc/foo
>
> expecting to get 0xffffffff?  That's a reasonable shorthand, and if we
> change that to spit out EINVAL then people's stuff may break.

If we expect the interface to allow overflows, we should at least add
comments to do_proc_dointvec_minmax_conv_param()...

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security