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

[Kairui Song]

On Wed, Sep 25, 2019 at 11:25 PM Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
>
> On Thu, 19 Sep 2019 at 18:06, Kairui Song <kasong@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
>
> 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