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

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Date: Mon, 3 Jun 2024 08:06:56 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Borislav Petkov <bp@...en8.de>, Mike Rapoport <rppt@...nel.org>
Cc: tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
 x86@...nel.org, rafael@...nel.org, hpa@...or.com, peterz@...radead.org,
 adrian.hunter@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
 jun.nakajima@...el.com, rick.p.edgecombe@...el.com, thomas.lendacky@....com,
 michael.roth@....com, seanjc@...gle.com, kai.huang@...el.com,
 bhe@...hat.com, kirill.shutemov@...ux.intel.com, bdas@...hat.com,
 vkuznets@...hat.com, dionnaglaze@...gle.com, anisinha@...hat.com,
 jroedel@...e.de, ardb@...nel.org, kexec@...ts.infradead.org,
 linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/3] efi/x86: Fix EFI memory map corruption with kexec

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?
>