Re: [PATCH v2 1/5] arm64: kexec_file: Forbid non-crash kernels

[James Morse]

On 31/05/2021 10:57, Marc Zyngier wrote:
> It has been reported that kexec_file doesn't really work on arm64.
> It completely ignores any of the existing reservations, which results
> in the secondary kernel being loaded where the GICv3 LPI tables live,


> or even corrupting the ACPI tables.

I'd like to know how the ACPI tables bit happens.

ACPI tables should be in EFI_ACPI_RECLAIM_MEMORY or EFI_ACPI_MEMORY_NVS (which isn't
treated as usable).

EFI's reserve_regions() does this:
|	if (!is_usable_memory(md))
|		memblock_mark_nomap(paddr, size);
|
|	/* keep ACPI reclaim memory intact for kexec etc. */
|	if (md->type == EFI_ACPI_RECLAIM_MEMORY)
|		memblock_reserve(paddr, size);

which is called via efi_init(), and all those regions end up listed as reserved in
/proc/iomem. (this is why arm64 doesn't call acpi_reserve_initial_tables())

If your firmware puts ACPI tables are in EFI_CONVENTIONAL_MEMORY, you have bigger problems
as the kernel could get relocated over the top of them during boot, and even if it
doesn't, nothing stops that  memory being allocated for user-space.

Even acpi_table_upgrade() calls memblock_reserve() and happens early enough not to be a
problem.


Please share ... enjoyment, optional.

(boot with efi=debug and post the EFI memory map and the 'ACPI: FOO 0xphysicaladdress'
stuff at the top of the boot log)


Thanks,

James


> Since only crash kernels are imune to this as they use a reserved
> memory region, disable the non-crash kernel use case. Further
> patches will try and restore the functionality.