Re: [PATCH v2] DRBG: fix maximum value checks on 32 bit systems

From: Stephan Mueller
Date: Tue Aug 26 2014 - 04:52:58 EST

Am Dienstag, 26. August 2014, 16:43:43 schrieb Herbert Xu:

Hi Herbert,

> On Tue, Aug 26, 2014 at 10:29:45AM +0200, Stephan Mueller wrote:
> > The maximum values for additional input string or generated blocks is
> > larger than 1<<32. To ensure a sensible value on 32 bit systems, return
> > SIZE_MAX on 32 bit systems. This value is lower than the maximum
> > allowed values defined in SP800-90A. The standard allow lower maximum
> > values, but not larger values.
> >
> > SIZE_MAX - 1 is used for drbg_max_addtl to allow
> > drbg_healthcheck_sanity to check the enforcement of the variable
> > without wrapping.
> This is really ugly but OK. However, I'm not sure how the sanity
> check ever worked. It would appear that the drbg_generate call in
> drbg_healthcheck_sanity should always fail because you explicitly
> set addtl->len to drbg_max_addtl + 1, which should trigger the
> "DRBG: additional information string too long" error, no?

That is exactly what the test shall do: the test is intended to check whether
the maximum values are enforced. And it does that by checking whether an error
is returned.

/* get the maximum value */
max_addtllen = drbg_max_addtl(drbg);
/* add one to definitely overflow the maximum value */
drbg_string_fill(&addtl, buf, max_addtllen + 1);
/* overflow addtllen with additonal info string */
len = drbg_generate(drbg, buf, OUTBUFLEN, &addtl);
* check that the drbg_generate does not return a positive
* value, i.e. check that drbg_generate does not generate anything
BUG_ON(0 < len);
> Obviously it's working for you but I'd like to understand why
> it's working and whether it'll continue to work.
> Thanks,

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at