Re: [patch 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v4

[H. Peter Anvin]

On 01/23/2012 06:20 AM, Cyrill Gorcunov wrote:
> +
> +static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
> +
> +static long kptr_obfuscate(long v, int type)
> +{
> +	return (v + cookies[type][0]) ^ cookies[type][1];
> +}
> +

Arf... when I said to use xor I meant instead of the add, not instead of
the multiply, so:

	return (v ^ cookies[type][0]) * cookies[type][1];

Otherwise you have absolutely no source of diffusion at all (symmetric
cryptography is about combinations of diffusion -- spreading the content
-- and confusion -- scrambling individual bits of content.)

+	for (i = 0; i < KCMP_TYPES; i++) {
+		for (j = 0; j < 2; j++) {
+			get_random_bytes(&cookies[i][j],
+					 sizeof(cookies[i][j]));
+			cookies[i][j] |= (~(~0UL >>  1) | 1);
+		}
+	}

Only cookies[1] -- being used as a multiplicative constant -- needs the OR.

	-hpa

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

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