linux-kernel - Re: [PATCH] x86/intel/quark: Remove lock bit around kernel IMR

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Message-ID: <CAHp75VeMxsux9pQ4P92=tDCSLqP93P3xacJYbRnV=kpe_=tR_w@mail.gmail.com>
Date:	Thu, 21 Jan 2016 20:44:13 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	"Bryan O'Donoghue" <pure.logic@...us-software.ie>
Cc:	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] x86/intel/quark: Remove lock bit around kernel IMR

On Thu, Jan 21, 2016 at 4:05 PM, Bryan O'Donoghue
<pure.logic@...us-software.ie> wrote:
> Currently when setting up an IMR around the kernel .text area we lock that
> IMR, preventing further modification. While superficially this appears to
> be the right thing to do, in fact this doesn't account for a legitimate
> change in the memory map such as when running through kexec. In such a
> scenario a second kernel can have a different size and location to it's
> predecessor and can view some of the memory occupied by its predecessor as
> legitimately usable RAM. This RAM can then be allocated to DMA agents
> within the system and trigger an IMR violation.
>
> The solution to this situation is to keep the kernel .text section IMR lock
> bit false. This means that a subsequent kernel will boot and can tear-down
> an existing IMR, while still setting up an IMR around its own .text
> section.
>

Like I said you privately it would be nice to have a knob how to behave.

My idea is to provide kernel command line imr= and supply imr=lock
when user wants to boot kernel locked.

Optionally: add a warning if imr=lock and kernel build with kexec that
might bring issues
Optionally: add a kernel config option to boot always in locked mode
(should depend on !KEXEC)

> Signed-off-by: Bryan O'Donoghue <pure.logic@...us-software.ie>
> Reported-by: Andriy Shevchenko <andriy.shevchenko@...el.com>
> ---
>  arch/x86/platform/intel-quark/imr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/platform/intel-quark/imr.c b/arch/x86/platform/intel-quark/imr.c
> index c1bdafa..1c43639 100644
> --- a/arch/x86/platform/intel-quark/imr.c
> +++ b/arch/x86/platform/intel-quark/imr.c
> @@ -591,7 +591,7 @@ static void __init imr_fixup_memmap(struct imr_device *idev)
>          * from the beginning of the .text secton to the end of the
>          * .rodata section as one physically contiguous block.
>          */
> -       ret = imr_add_range(base, size, IMR_CPU, IMR_CPU, true);
> +       ret = imr_add_range(base, size, IMR_CPU, IMR_CPU, false);
>         if (ret < 0) {
>                 pr_err("unable to setup IMR for kernel: (%p - %p)\n",
>                         &_text, &__end_rodata);
> --
> 2.5.0
>



-- 
With Best Regards,
Andy Shevchenko