linux-kernel - Re: [PATCH] arm64: add support for rng-seed

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Date:   Wed, 8 May 2019 09:04:10 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Hsin-Yi Wang <hsinyi@...omium.org>
Cc:     "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Frank Rowand <frowand.list@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        James Morse <james.morse@....com>,
        Andrew Murray <andrew.murray@....com>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Architecture Mailman List <boot-architecture@...ts.linaro.org>
Subject: Re: [PATCH] arm64: add support for rng-seed

On Tue, May 7, 2019 at 11:08 PM Hsin-Yi Wang <hsinyi@...omium.org> wrote:
>
> On Wed, May 8, 2019 at 3:47 AM Rob Herring <robh+dt@...nel.org> wrote:
> >
> > +boot-architecture list as there was some discussion about this IIRC.
> >
> > On Mon, May 6, 2019 at 11:54 PM Hsin-Yi Wang <hsinyi@...omium.org> wrote:
> > >
> > > Introducing a chosen node, rng-seed, which is an 64 bytes entropy
> > > that can be passed to kernel called very early to increase device
> > > randomness. Bootloader should provide this entropy and the value is
> > > read from /chosen/rng-seed in DT.
> > >
> > > Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> > >
> > > ---
> > >  Documentation/devicetree/bindings/chosen.txt | 14 +++++++++
> >
> > Actually, this file has been converted to json-schema and lives
> > here[1]. I need to remove this one (or leave it with a reference to
> > the new one).
> >
> > >  arch/arm64/kernel/setup.c                    |  2 ++
> > >  drivers/of/fdt.c                             | 33 ++++++++++++++++++++
> > >  include/linux/of_fdt.h                       |  1 +
> > >  4 files changed, 50 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
> > > index 45e79172a646..bfd360691650 100644
> > > --- a/Documentation/devicetree/bindings/chosen.txt
> > > +++ b/Documentation/devicetree/bindings/chosen.txt
> > > @@ -28,6 +28,20 @@ mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
> > >  the Linux EFI stub (which will populate the property itself, using
> > >  EFI_RNG_PROTOCOL).
> > >
> > > +rng-seed
> > > +-----------
> > > +
> > > +This property served as an entropy to add device randomness. It is parsed
> > > +as a 64 byte value, e.g.
> >
> > Why only 64-bytes?
> We can also not specify size and read what bootloader can provide.
> >
> > > +
> > > +/ {
> > > +       chosen {
> > > +               rng-seed = <0x31951b3c 0xc9fab3a5 0xffdf1660 ...>
> > > +       };
> > > +};
> > > +
> > > +This random value should be provided by bootloader.
> > > +
> > >  stdout-path
> > >  -----------
> > >
> > > diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> > > index 413d566405d1..ade4261516dd 100644
> > > --- a/arch/arm64/kernel/setup.c
> > > +++ b/arch/arm64/kernel/setup.c
> > > @@ -292,6 +292,8 @@ void __init setup_arch(char **cmdline_p)
> > >         early_fixmap_init();
> > >         early_ioremap_init();
> > >
> > > +       early_init_dt_rng_seed(__fdt_pointer);
> > > +
> >
> > I'm trying to reduce or eliminate all these early_init_dt_* calls.
> >
> > Why is this arch specific and why can't this be done after
> > unflattening? It doesn't look like add_device_randomness() needs
> > anything early.
> Currently unflattening is called after setup_machine_fdt(), which
> called fixmap_remap_fdt() //__fixmap_remap_fdt(dt_phys, &size,
> PAGE_KERNEL_RO), and we can't modify DT after that since it's read
> only. But we need to clear (eg. write 0 to it) the rng-seed after
> reading from DT.

Why do you need to clear it? That wasn't necessary for kaslr-seed.

Why not change the mapping to RW? It would be nice if this worked on
more than one arch.

Rob