linux-kernel - Re: [PATCH v3] x86, efi: never relocate kernel below lowest acceptable address

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Date:   Mon, 14 Oct 2019 12:14:19 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Kairui Song <kasong@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
        x86@...nel.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v3] x86, efi: never relocate kernel below lowest
 acceptable address

On Sat, Oct 12, 2019 at 11:44:21AM +0800, Kairui Song wrote:
> Currently, kernel fails to boot on some HyperV VMs when using EFI.
> And it's a potential issue on all platforms.
> 
> It's caused a broken kernel relocation on EFI systems, when below three
> conditions are met:
> 
> 1. Kernel image is not loaded to the default address (LOAD_PHYSICAL_ADDR)
>    by the loader.
> 2. There isn't enough room to contain the kernel, starting from the
>    default load address (eg. something else occupied part the region).
> 3. In the memmap provided by EFI firmware, there is a memory region
>    starts below LOAD_PHYSICAL_ADDR, and suitable for containing the
>    kernel.
> 
> Efi stub will perform a kernel relocation when condition 1 is met. But
> due to condition 2, efi stub can't relocate kernel to the preferred
> address, so it fallback to query and alloc from EFI firmware for lowest

Your spelling of "EFI" is like a random number generator in this
paragraph: "Efi", "efi" and "EFI". Can you please be more careful when
writing your commit messages? They're not some random text you hurriedly
jot down before sending the patch but a most important description of
why a change is being done.

And if you don't see their importance now, just try doing some git
archeology, trying to understand why a change has been done in the past
and then encounter a commit message two-liner which doesn't say sh*t.
Then you'll start appreciating properly written commit messages.

> usable memory region.
> 
> It's incorrect to use the lowest memory address. In later stage, kernel
> will assume LOAD_PHYSICAL_ADDR as the minimal acceptable relocate address,
> but efi stub will end up relocating kernel below it.

Why don't you simply explain what
choose_random_location()->find_random_virt_addr() does? That's the
problem you're solving, right? KASLR using LOAD_PHYSICAL_ADDR as the
minimum...

> The later kernel decompressing code will forcefully correct the wrong
> kernel load location,

... or do you mean by that the dance in
arch/x86/boot/compressed/head_64.S where we move the kernel temporarily
to LOAD_PHYSICAL_ADDR for the decompression?

You can simply say that here...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette