linux-kernel - Re: [kbuild-all] Re: [PATCH v43 01/15] Linux Random Number Generator

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Date:   Tue, 30 Nov 2021 07:06:11 +0100
From:   Stephan Müller <smueller@...onox.de>
To:     "Chen, Rong A" <rong.a.chen@...el.com>,
        Sandy Harris <sandyinchina@...il.com>
Cc:     Tso Ted <tytso@....edu>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        kernel test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        Willy Tarreau <w@....eu>, Nicolai Stange <nstange@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>
Subject: Re: [kbuild-all] Re: [PATCH v43 01/15] Linux Random Number Generator

Am Dienstag, 30. November 2021, 03:55:12 CET schrieb Sandy Harris:

Hi Sandy,

> Chen, Rong A <rong.a.chen@...el.com> wrote:
> > On 11/22/2021 7:47 PM, Stephan Mueller wrote:
> > > Thanks for the notification.
> > > 
> > > I think this is a false-positive discussed before. __latent_entropy is
> > > seemingly allowed for an entire linear buffer as seen in the declaration
> > > of
> > > the variable input_pool_data in driver/char/random.c which is an array
> > > of u32.
> > > 
> > > The struct chacha20_state is a linear buffer of u32 words.
> > > 
> > > struct chacha20_block {
> > > 
> > >          u32 constants[4];
> > >          union {
> > >          
> > >                  u32 u[CHACHA_KEY_SIZE_WORDS];
> > >                  u8  b[CHACHA_KEY_SIZE];
> > >          
> > >          } key;
> > >          u32 counter;
> > >          u32 nonce[3];
> > > 
> > > };
> > > 
> > > Therefore it should be identical to the aforementioned example.
> 
> No. It is a struct & there's no guarantee all compilers will lay
> it out as you  expect. There might even be a gap in the layout
> since nonce[] has an odd number of elements.
> 
> >> The __latent_entropy marker therefore seems to be appropriate for this
> >> structure.
> First, this is completely unnecessary since the input pool is marked for
> latent entropy & changes there will affect the chacha context.
> 
> Also, if I'm reading the docs right, the __latent_entropy attribute
> on a data structure only gets it initialised somewhat randomly.
> If you want a continuous effect at runtime, then you need to
> make the code mix the latent_entropy global variable into the
> data structure.

Thank you very much for your explanation. I will change my code accordingly.

Note, the LRNG does not have an input_pool.

Ciao
Stephan