Re: [PATCH v2 5/6] crypto: caam - replace DRNG with TRNG for use with hw_random

[Andrey Smirnov]

On Mon, Nov 18, 2019 at 7:50 AM Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
>
> On Mo, 2019-11-18 at 07:38 -0800, Andrey Smirnov wrote:
> > In order to give CAAM-generated random data highest quarlity
> > raiting (999), replace current code that uses DRNG with code that
> > fetches data straight out of TRNG used to seed aforementioned DRNG.
> >
> > Signed-off-by: Andrey Smirnov <andrew.smirnov@xxxxxxxxx>
> > Cc: Chris Healy <cphealy@xxxxxxxxx>
> > Cc: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> > Cc: Horia GeantÄ <horia.geanta@xxxxxxx>
> > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> > Cc: Iuliana Prodan <iuliana.prodan@xxxxxxx>
> > Cc: linux-imx@xxxxxxx
> > Cc: linux-crypto@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> [...]
> > diff --git a/drivers/crypto/caam/trng.c b/drivers/crypto/caam/trng.c
> > new file mode 100644
> > index 000000000000..ab2af786543e
> > --- /dev/null
> > +++ b/drivers/crypto/caam/trng.c
> > @@ -0,0 +1,85 @@
> > +// SPDX-License-Identifier: GPL-2.0+
> > +/*
> > + * hw_random interface for TRNG generator in CAAM RNG block
> > + *
> > + * Copyright 2019 Zoidac Inflight Innovations
>                      ^ Zodiac
>

Ugh, thanks for catching this, will fix in v3

> > + *
> > + */
> > +
> > +#include <linux/hw_random.h>
> > +
> > +#include "compat.h"
> > +#include "regs.h"
> > +#include "intern.h"
> > +
> > +struct caam_trng_ctx {
> > +     struct rng4tst __iomem *r4tst;
> > +     struct hwrng rng;
> > +};
> > +
> > +static bool caam_trng_busy(struct caam_trng_ctx *ctx)
> > +{
> > +     return !(rd_reg32(&ctx->r4tst->rtmctl) & RTMCTL_ENT_VAL);
> > +}
> > +
> > +static int caam_trng_read(struct hwrng *rng, void *data, size_t max, bool wait)
> > +{
> > +     struct caam_trng_ctx *ctx = (void *)rng->priv;
> > +     u32 rtent[ARRAY_SIZE(ctx->r4tst->rtent)];
> > +     size_t residue = max;
> > +
> > +     clrsetbits_32(&ctx->r4tst->rtmctl, 0, RTMCTL_ACC);
> > +
> > +     do {
> > +             const size_t chunk = min(residue, sizeof(rtent));
> > +             unsigned int i;
> > +
> > +             while (caam_trng_busy(ctx)) {
>
> The CAAM needs quite a bit of time to gather the 384bits of raw
> entropy, in my testing it was almost 60ms. A busy loop (even with a
> cpu_relax) for such an extended amount of time is probably not
> appropriate, better sleep for some time here.
>

Good point, will fix in v3.

> Also in the !wait case we are almost guaranteed to leave this function
> without any entropy gathered. Maybe we should just bail out on !wait
> without even trying to enable the TRNG access?
>

Yeah, I think you're right. Will change in v3.

Thanks,
Andrey Smirnov