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

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Date:   Tue, 30 Nov 2021 10:55:12 +0800
From:   Sandy Harris <sandyinchina@...il.com>
To:     "Chen, Rong A" <rong.a.chen@...el.com>
Cc:     Stephan Mueller <smueller@...onox.de>, 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

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.