linux-kernel - Re: [PATCH v3 2/3] arm64: implement update_fdt

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Date:   Thu, 16 May 2019 10:44:28 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Hsin-Yi Wang <hsinyi@...omium.org>,
        James Morse <james.morse@....com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Miles Chen <miles.chen@...iatek.com>,
        Andrew Murray <andrew.murray@....com>,
        Mark Rutland <mark.rutland@....com>,
        Jun Yao <yaojun8558363@...il.com>, Yu Zhao <yuzhao@...gle.com>,
        Robin Murphy <robin.murphy@....com>,
        Laura Abbott <labbott@...hat.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 2/3] arm64: implement update_fdt_pgprot()

Quoting James Morse (2019-05-16 10:34:16)
> Hi!
> 
> On 16/05/2019 17:48, Hsin-Yi Wang wrote:
> > On Thu, May 16, 2019 at 11:32 PM Rob Herring <robh+dt@...nel.org> wrote:
> >> Doesn't kexec operate on a copy because it already does modifications.
> 
> It does!
> 
> > This patch is to assist "[PATCH v3 3/3] fdt: add support for rng-seed"
> > (https://lkml.org/lkml/2019/5/16/257). I thought that by default
> > second kernel would use original fdt, so I write new seed back to
> > original fdt. Might be wrong.
> > 
> > ** "[PATCH v3 3/3] fdt: add support for rng-seed" is supposed to
> > handle for adding new seed in kexec case, discussed in v2
> > (https://lkml.org/lkml/2019/5/13/425)
> > 
> > By default (not considering user defines their own fdt), if second
> > kernel uses copied fdt, when is it copied and can we modify that?
> 
> Regular kexec's user-space already updates the dtb for the cmdline and maybe the initrd.
> For KASLR, it generates its own seed with getrandom():
> 
> https://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git/tree/kexec/arch/arm64/kexec-arm64.c#n483
> 
> If user-space can do it, user-space should do it!
> 

Doesn't it need to be done in two places? Userspace and also in the
kernel when kexec_file_load() is used? At least, I see a bit of code
that does kaslr seed updates to the copied dtb in setup_dtb() of
arch/arm64/kernel/machine_kexec_file.c that probably needs to get an
update for this new property too.