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

[Mike Rapoport <rppt@kernel.org>]

On Mon, Jun 03, 2024 at 09:01:49AM -0500, Kalra, Ashish wrote:
> On 6/3/2024 8:39 AM, Mike Rapoport wrote:
> 
> > 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?
> 
> This is observed under SNP as efi_arch_mem_reserve() is only being called
> with SNP enabled and then efi_arch_mem_reserve() allocates EFI memory map
> using memblock.

I don't see how efi_arch_mem_reserve() is only called with SNP. What did I
miss?
 
> If we skip efi_arch_mem_reserve() (which should probably be anyway skipped
> for kexec case), then for kexec boot, EFI memmap is memremapped in the same
> virtual address as the first kernel and not the allocated memblock address.

Maybe we should skip efi_arch_mem_reserve() for kexec case, but I think we
still need to understand what's causing memory corruption.

> Thanks, Ashish
> 
> > 
> > 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.