linux-kernel - Re: [PATCH v4] x86, efi: never relocate kernel below lowest acceptable address

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Date:   Tue, 22 Oct 2019 09:44:22 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Kairui Song <kasong@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.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>,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v4] x86, efi: never relocate kernel below lowest
 acceptable address

On Tue, Oct 22, 2019 at 08:13:56AM +0200, Ard Biesheuvel wrote:
> On Thu, 17 Oct 2019 at 11:30, Kairui Song <kasong@...hat.com> 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 by 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 ask EFI firmware to alloc lowest usable memory
> > region, got the low region mentioned in condition 3, and relocated
> > kernel there.
> >
> > It's incorrect to relocate the kernel below LOAD_PHYSICAL_ADDR. This
> > is the lowest acceptable kernel relocation address.
> >
> > The first thing goes wrong is in arch/x86/boot/compressed/head_64.S.
> > Kernel decompression will force use LOAD_PHYSICAL_ADDR as the output
> > address if kernel is located below it. Then the relocation before
> > decompression, which move kernel to the end of the decompression buffer,
> > will overwrite other memory region, as there is no enough memory there.
> >
> > To fix it, just don't let EFI stub relocate the kernel to any address
> > lower than lowest acceptable address.
> >
> > Signed-off-by: Kairui Song <kasong@...hat.com>
> > Acked-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> >
> 
> Ingo, Boris, could you please comment on this?

Yah, the commit message makes more sense now.

Thx.

-- 
Regards/Gruss,
    Boris.

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