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

[Kairui Song]

On Fri, Oct 11, 2019 at 9:24 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> 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.
>
> So far, so good.
>
> > 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.
>
> This paragraph is an unreadable mess and should be rewritten in simple,
> declarative sentences.
>
> The patch itself looks ok.

Thanks. I've sent V3 updating that paragraph.

>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
--
Best Regards,
Kairui Song