linux-kernel - Re: [PATCH 4/5] x86/boot/64: Explicitly map boot

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Date:   Thu, 8 Oct 2020 09:57:44 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] x86/boot/64: Explicitly map boot_params and command
 line

On Thu, Oct 08, 2020 at 11:48:36AM +0200, Joerg Roedel wrote:
> On Wed, Oct 07, 2020 at 03:53:50PM -0400, Arvind Sankar wrote:
> > This is fragile, as boot_params and the command line mappings are
> > required for the main kernel. If EARLY_PRINTK and RANDOMIZE_BASE are
> > disabled, a QEMU/OVMF boot never accesses the command line in the
> > decompressor stub, and so it never gets mapped. The main kernel accesses
> > it from the identity mapping if AMD_MEM_ENCRYPT is enabled, and will
> > crash.
> 
> Looked again, and I think that is wrong for boot_params, which are
> touched unconditionally at the beginning of extract_kernel().

Yes, command line is the only thing that actually breaks, but it is more
robust to explicitly make sure boot_params is mapped as well. There's no
specific alignment requirement for boot_params AFAICT, so at least in
theory it's possible that it would be split across a PMD boundary and
only get half-mapped in the decompressor. It's easier not to have to
worry about it.

> 
> For the cmdline you are right, but one of CONFIG_ACPI,
> CONFIG_RANDOMIZE_BASE, CONFIG_X86_5LEVEL or CONFIG_EARLY_PRINTK is
> sufficient to have it touched during this boot stage.
> 

X86_5LEVEL accesses it before the switch to the new page tables, so that
doesn't help in getting it mapped. ACPI only accesses it if KASLR is
enabled (as well as MEMORY_HOTREMOVE).

Thanks.