linux-kernel - Re: [PATCH 0/3] Resolve problems with kexec identity mapping

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Message-ID: <CAMj1kXGjHFarKyeT8jyan=pX58CeLisOqSnU_0V+2j04RC00Rg@mail.gmail.com>
Date: Wed, 3 Jul 2024 18:18:38 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Steve Wahl <steve.wahl@....com>, Ashish Kalra <ashish.kalra@....com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	linux-kernel@...r.kernel.org, Pavin Joseph <me@...injoseph.com>, 
	Eric Hagberg <ehagberg@...il.com>, Simon Horman <horms@...ge.net.au>, 
	Eric Biederman <ebiederm@...ssion.com>, Dave Young <dyoung@...hat.com>, Sarah Brofeldt <srhb@....dk>, 
	Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>, 
	Hou Wenlong <houwenlong.hwl@...group.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Yuntao Wang <ytcoode@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Joerg Roedel <jroedel@...e.de>, Michael Roth <michael.roth@....com>
Subject: Re: [PATCH 0/3] Resolve problems with kexec identity mapping

On Wed, 3 Jul 2024 at 18:15, Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Jul 02, 2024 at 08:32:22PM +0200, Ard Biesheuvel wrote:
> > For kexec on a 64-bit system, I would expect the high-level support
> > code to be capable of simply mapping all of DRAM 1:1, rather than
> > playing these games with #PF handlers and on-demand mapping.
>
> Yeah, apparently we can't do that on SGI, as Steve said.
>
> I like the aspect that the #PF handler won't fire in the first kernel because
> of EFI mapping all RAM. That's good.
>

It won't fire because the code where this handler is being added is
never even called by EFI boot - it decompresses the kernel from the
EFI stub and jumps straight to its entrypoint.


> So we could try to wire in a #PF handler in stage1, see below.
>

Looks fine to me from EFI boot pov, for the reasons given above.

> Steve, I don't have a good idea how to test that. Maybe some of those
> reporters you were talking about, would be willing to...
>
> ---
> diff --git a/arch/x86/boot/compressed/idt_64.c b/arch/x86/boot/compressed/idt_64.c
> index d100284bbef4..a258587c8949 100644
> --- a/arch/x86/boot/compressed/idt_64.c
> +++ b/arch/x86/boot/compressed/idt_64.c
> @@ -32,6 +32,7 @@ void load_stage1_idt(void)
>  {
>         boot_idt_desc.address = (unsigned long)boot_idt;
>
> +       set_idt_entry(X86_TRAP_PF, boot_page_fault);
>
>         if (IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
>                 set_idt_entry(X86_TRAP_VC, boot_stage1_vc);
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette