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

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Date:   Fri, 11 Oct 2019 18:18:40 +0800
From:   Kairui Song <kasong@...hat.com>
To:     "the arch/x86 maintainers" <x86@...nel.org>
Cc:     Linux Kernel Mailing List <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>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v2] x86, efi: never relocate kernel below lowest
 acceptable address

On Thu, Sep 26, 2019 at 1:36 AM Kairui Song <kasong@...hat.com> wrote:
>
> On Wed, Sep 25, 2019 at 11:25 PM Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
> >
> > On Thu, 19 Sep 2019 at 18:06, 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 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>
> > >
> >
> > Hello Kairui,
> >
> > This patch looks correct to me, but it needs an ack from the x86
> > maintainers, since the rules around LOAD_PHYSICAL_ADDR are specific to
> > the x86 architecture.
> >
> >
>
> Thanks for the review, Ard.
>
> Can any x86 maintainer help provide some review?
>
> --
> Best Regards,
> Kairui Song

Ping? Any comments?

--
Best Regards,
Kairui Song