linux-kernel - Re: [PATCH 6/8] ASoC: intel: skylake: change snprintf to scnprintf for possible overflow

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Date:   Wed, 16 Jan 2019 13:35:51 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Kees Cook <keescook@...omium.org>, Willy Tarreau <w@....eu>
Cc:     Silvio Cesare <silvio.cesare@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Liam Girdwood <liam.r.girdwood@...ux.intel.com>,
        Jie Yang <yang.jie@...ux.intel.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Will Deacon <will.deacon@....com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 6/8] ASoC: intel: skylake: change snprintf to scnprintf
 for possible overflow


>> diff --git a/sound/soc/intel/skylake/skl-debug.c b/sound/soc/intel/skylake/skl-debug.c
>> index 5d7ac2ee7a3c..bb28db734fb7 100644
>> --- a/sound/soc/intel/skylake/skl-debug.c
>> +++ b/sound/soc/intel/skylake/skl-debug.c
>> @@ -43,7 +43,7 @@ static ssize_t skl_print_pins(struct skl_module_pin *m_pin, char *buf,
>>          ssize_t ret = 0;
>>
>>          for (i = 0; i < max_pin; i++)
>> -               ret += snprintf(buf + size, MOD_BUF - size,
>> +               ret += scnprintf(buf + size, MOD_BUF - size,
>>                                  "%s %d\n\tModule %d\n\tInstance %d\n\t"
>>                                  "In-used %s\n\tType %s\n"
>>                                  "\tState %d\n\tIndex %d\n",
>>
> While working on a Coccinelle script to find more cases of this, I
> noticed that this code is buggy: it keeps overwriting the same
> position in the buf string: "buf + size" and don't take "ret" into
> account at all. This needs to be:
>
> ret += scnprintf(buf + size + ret, MOD_BUF - size - ret,

Thanks for the sighting. Indeed this looks like a bug, all other calls 
to snprintf use "ret" to modify the destination/length.

The only explanation I have for it not being noticed earlier is that 
it's possibly not used - a 5mn test on 2 machines show the loop is 
actually not run (max_pin == 0).

It'll take me a bit of time to figure out what exactly this routine is 
supposed to do, maybe we should do the cross-tree change first?

-Pierre