linux-kernel - Re: [PATCH v12] ARM: uncompress: Validate start of physical memory against passed DTB

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Date:   Sat, 9 Jan 2021 00:57:24 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Russell King <linux@...linux.org.uk>,
        Ard Biesheuvel <ardb@...nel.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Miao <eric.miao@...dia.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Lukasz Stelmach <l.stelmach@...sung.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Chris Brandt <chris.brandt@...esas.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12] ARM: uncompress: Validate start of physical memory
 against passed DTB

On Mon, Jan 4, 2021 at 2:01 PM Geert Uytterhoeven
<geert+renesas@...der.be> wrote:

> Currently, the start address of physical memory is obtained by masking
> the program counter with a fixed mask of 0xf8000000.  This mask value
> was chosen as a balance between the requirements of different platforms.
> However, this does require that the start address of physical memory is
> a multiple of 128 MiB, precluding booting Linux on platforms where this
> requirement is not fulfilled.
>
> Fix this limitation by validating the masked address against the memory
> information in the passed DTB.  Only use the start address
> from DTB when masking would yield an out-of-range address, prefer the
> traditional method in all other cases.  Note that this applies only to the
> explicitly passed DTB on modern systems, and not to a DTB appended to
> the kernel, or to ATAGS.  The appended DTB may need to be augmented by
> information from ATAGS, which may need to rely on knowledge of the start
> address of physical memory itself.
>
> This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM
> on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space),
> i.e. not at a multiple of 128 MiB.
>
> Suggested-by: Nicolas Pitre <nico@...xnic.net>
> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> Acked-by: Nicolas Pitre <nico@...xnic.net>

Sorry for the long delay in reading the patch :(

I really like the looks of this now, moreover it is very useful.
I suppose it is already in the patch tracker, but for the record:
Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

> +               reg = fdt_getprop(fdt, offset, "linux,usable-memory", &len);

I suppose we already had a discussion of why this property
is undocumented? Or should we document it? Obviously
it is already in widespread use.

Yours,
Linus Walleij