Subject: Re: [PATCH] powerpc/vio: Fix modalias_show return values

From: Prarit Bhargava <[email-masked]>
Date: Thu, 17 Oct 2013 07:50:23 -0400

On 10/16/2013 11:53 PM, Ben Hutchings wrote:
> Commit e82b89a6f19bae73fb064d1b3dd91fcefbb478f4 introduces a trivial
> local denial of service.
> 
>> --- a/arch/powerpc/kernel/vio.c
>> +++ b/arch/powerpc/kernel/vio.c
>> @@ -1351,11 +1351,15 @@ static ssize_t modalias_show(struct devi
>>  	const char *cp;
>>  
>>  	dn = dev->of_node;
>> -	if (!dn)
>> -		return -ENODEV;
>> +	if (!dn) {
>> +		strcat(buf, "\n");
> 
> Every read from the same sysfs file handle uses the same buffer, which
> gets zero-initialised just once.  So if I open the file, read it and
> seek back to 0 repeatedly, I can make modalias_show() write arbitrary
> numbers of newlines into *and beyond* that page-sized buffer.
> 
> Obviously strcat() should be strcpy().
> 

D'oh!  Of course -- I wasn't thinking clearly about that.  I'll send out a new
patch.

P.

> Ben.
> 
>> +		return strlen(buf);
>> +	}
>>  	cp = of_get_property(dn, "compatible", NULL);
>> -	if (!cp)
>> -		return -ENODEV;
>> +	if (!cp) {
>> +		strcat(buf, "\n");
>> +		return strlen(buf);
>> +	}
>>  
>>  	return sprintf(buf, "vio:T%sS%s\n", vio_dev->type, cp);
>>  }
>