Re: [PATCH v7 1/3] efi/x86: Fix EFI memory map corruption with kexec

[Kalra, Ashish]

On 6/3/2024 3:56 AM, Borislav Petkov wrote

> > EFI memory map and due to early allocation it uses memblock allocation.
> >
> > Later during boot, efi_enter_virtual_mode() calls kexec_enter_virtual_mode()
> > in case of a kexec-ed kernel boot.
> >
> > This function kexec_enter_virtual_mode() installs the new EFI memory map by
> > calling efi_memmap_init_late() which remaps the efi_memmap physically allocated
> > in efi_arch_mem_reserve(), but this remapping is still using memblock allocation.
> >
> > Subsequently, when memblock is freed later in boot flow, this remapped
> > efi_memmap will have random corruption (similar to a use-after-free scenario).
> >
> > The corrupted EFI memory map is then passed to the next kexec-ed kernel
> > which causes a panic when trying to use the corrupted EFI memory map.
> This sounds fishy: memblock allocated memory is not freed later in the
> boot - it remains reserved. Only free memory is freed from memblock to
> the buddy allocator.
>
> Or is the problem that memblock-allocated memory cannot be memremapped
> because *raisins*?

This is what seems to be happening:

efi_arch_mem_reserve() calls efi_memmap_alloc() to allocate memory for
EFI memory map and due to early allocation it uses memblock allocation.

And later efi_enter_virtual_mode() calls kexec_enter_virtual_mode()
in case of a kexec-ed kernel boot.

This function kexec_enter_virtual_mode() installs the new EFI memory map by
calling efi_memmap_init_late() which does memremap() on memblock-allocated memory.

Thanks, Ashish

> Mike?