linux-kernel - Re: [PATCHv2 2/2] hwrng: optee: fix wait use case

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Date:   Fri, 24 Jul 2020 16:23:05 +0200
From:   "Jorge Ramirez-Ortiz, Foundries" <jorge@...ndries.io>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Jorge Ramirez-Ortiz <jorge@...ndries.io>,
        Matt Mackall <mpm@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, ricardo@...ndries.io,
        Michael Scott <mike@...ndries.io>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        op-tee@...ts.trustedfirmware.org,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 2/2] hwrng: optee: fix wait use case

On 24/07/20, Sumit Garg wrote:
> On Thu, 23 Jul 2020 at 14:16, Jorge Ramirez-Ortiz <jorge@...ndries.io> wrote:
> >
> > The current code waits for data to be available before attempting a
> > second read. However the second read would not be executed as the
> > while loop exits.
> >
> > This fix does not wait if all data has been read and reads a second
> > time if only partial data was retrieved on the first read.
> >
> > This fix also does not attempt to read if not data is requested.
> 
> I am not sure how this is possible, can you elaborate?

currently, if the user sets max 0, get_optee_rng_data will regardless
issuese a call to the secure world requesting 0 bytes from the RNG

with this patch, this request is avoided.

> 
> >
> > Signed-off-by: Jorge Ramirez-Ortiz <jorge@...ndries.io>
> > ---
> >  v2: tidy up the while loop to avoid reading when no data is requested
> >
> >  drivers/char/hw_random/optee-rng.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/char/hw_random/optee-rng.c b/drivers/char/hw_random/optee-rng.c
> > index 5bc4700c4dae..a99d82949981 100644
> > --- a/drivers/char/hw_random/optee-rng.c
> > +++ b/drivers/char/hw_random/optee-rng.c
> > @@ -122,14 +122,14 @@ static int optee_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
> >         if (max > MAX_ENTROPY_REQ_SZ)
> >                 max = MAX_ENTROPY_REQ_SZ;
> >
> > -       while (read == 0) {
> > +       while (read < max) {
> >                 rng_size = get_optee_rng_data(pvt_data, data, (max - read));
> >
> >                 data += rng_size;
> >                 read += rng_size;
> >
> >                 if (wait && pvt_data->data_rate) {
> > -                       if (timeout-- == 0)
> > +                       if ((timeout-- == 0) || (read == max))
> 
> If read == max, would there be any sleep?

no but I see no reason why there should be a wait since we already have
all the data that we need; the msleep is only required when we need to
wait for the RNG to generate entropy for the number of bytes we are
requesting. if we are requesting 0 bytes, the entropy is already
available. at leat this is what makes sense to me.


> 
> -Sumit
> 
> >                                 return read;
> >                         msleep((1000 * (max - read)) / pvt_data->data_rate);
> >                 } else {
> > --
> > 2.17.1
> >