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Date: Mon, 3 Jun 2024 16:39:10 +0300
From: Mike Rapoport <rppt@kernel.org>
To: "Kalra, Ashish" <ashish.kalra@amd.com>
Cc: Borislav Petkov <bp@alien8.de>, tglx@linutronix.de, mingo@redhat.com,
	dave.hansen@linux.intel.com, x86@kernel.org, rafael@kernel.org,
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Subject: Re: [PATCH v7 1/3] efi/x86: Fix EFI memory map corruption with kexec
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On Mon, Jun 03, 2024 at 08:06:56AM -0500, Kalra, Ashish wrote:
> 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.

Does the issue happen only with SNP?

I didn't really dig, but my theory would be that it has something to do
with arch_memremap_can_ram_remap() in arch/x86/mm/ioremap.c
 
> Thanks, Ashish

-- 
Sincerely yours,
Mike.