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

[Rob Herring <robh+dt@kernel.org>]

On Wed, May 8, 2019 at 10:06 AM Hsin-Yi Wang <hsinyi@chromium.org> wrote:
>
> On Wed, May 8, 2019 at 10:04 PM Rob Herring <robh+dt@kernel.org> wrote:
> >
> > On Tue, May 7, 2019 at 11:08 PM Hsin-Yi Wang <hsinyi@chromium.org> wrote:
> > >
> > > On Wed, May 8, 2019 at 3:47 AM Rob Herring <robh+dt@kernel.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@chromium.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@chromium.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.
> I think it's for security purpose. If we know the random seed, it's
> more likely we can predict randomness.
> Currently on arm64, kaslr-seed will be wiped out (in
> arch/arm64/kernel/kaslr.c#get_kaslr_seed(), it's set to 0) so we can't
> read from sysfs (eg. /sys/firmware/devicetree/.../kaslr-seed)
> I'm not sure on other arch if it will be wiped out.

The difference is if I have the kaslr seed, I can calculate the kernel
base address.

In your case, you are feeding an RNG which continually has entropy
added to it. I can't see that knowing one piece of the entropy data is
a security hole. It looks more like you've just copied what what done
for kaslr-seed.

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

Still wondering on this question. Mapping it R/W would mean rng-seed
could be handled later and completely out of the arch code and so
could the zeroing of the kaslr-seed. Also, we generally assume the FDT
is modifiable for any fixups. This happens on arm32 and powerpc, but I
guess we haven't needed that yet on arm64.

Rob