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

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Date:	Tue, 14 Jun 2016 14:05:33 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Heinrich Schuchardt <xypron.glpk@....de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Don Zickus <dzickus@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Dave Young <dyoung@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Cashman <dcashman@...gle.com>, Willy Tarreau <w@....eu>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ilya Dryomov <idryomov@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] kernel/sysctl.c: avoid overflow

On Tue, Jun 14, 2016 at 1:19 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Sat, 11 Jun 2016 03:33:08 +0200 Heinrich Schuchardt <xypron.glpk@....de> 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