linux-kernel - Re: [PATCH v2] x86, efi: never relocate kernel below lowest acceptable address

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Date:   Wed, 25 Sep 2019 12:51:14 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
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>, Borislav Petkov <bp@...en8.de>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
        x86@...nel.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v2] x86, efi: never relocate kernel below lowest
 acceptable address

On Fri, Sep 20, 2019 at 12:05:21AM +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
> 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.
> 
> Then before the kernel decompressing. Kernel will do another relocation
> to address not lower than LOAD_PHYSICAL_ADDR, this time the relocate will
> over write the blockage at the default load address, which efi stub tried
> to avoid, and lead to unexpected behavior. Beside, the memory region it
> writes to is not allocated from EFI firmware, which is also wrong.
> 
> 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@...ux.intel.com>

/Jarkko