Re: [RFC] syscalls, x86: Add __NR_kcmp syscall

["H. Peter Anvin" <hpa@zytor.com>]

On 01/20/2012 01:02 AM, Cyrill Gorcunov wrote:
> On Fri, Jan 20, 2012 at 12:40:08PM +0400, Cyrill Gorcunov wrote:
> ...
>> +
>> +static __init int kcmp_cookie_init(void)
>> +{
>> +	int i, j;
>> +
>> +	for (i = 0; i < KCMP_TYPES; i++) {
>> +		for (j = 0; j < 2; j++) {
>> +			get_random_bytes(&cookies[i][j], sizeof(cookies[i][j]));
>> +
>> +			if (cookies[i][j])
>> +				continue;
>> +
>> +			/*
>> +			 * This is impossible case but just to be sure.
>> +			 */
>> +			cookies_valid = false;
>> +			WARN_ONCE(1, "Can't get random bytes for k-pointers\n");
>> +		}
>> +	}
>> +
>> +	cookies_valid = true;
> 
> darn, this string of course should be on top,
> i'll update don't complain on this nit.
> 

This code is wrong.  You will have a zero cookie, legitimately, once in
2^32 or 2^64 attempts, depending on the bitness.

The other thing is that for the multiplicative cookie you should OR in
the value (~(~0UL >> 1) | 1) in order to make sure that the value is (a)
large and (b) odd.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.