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

Tue Jun 4 15:35:57 PDT 2024

Re-sending as the earlier response got line-wrapped.

On 6/3/2024 12:12 PM, Borislav Petkov wrote:
> On Mon, Jun 03, 2024 at 12:08:48PM -0500, Kalra, Ashish wrote:
>> efi_arch_mem_reserve().
> Now it only remains for you to explain why...

Here is a detailed explanation of what is causing the EFI memory map corruption, with added debug logs and memblock debugging enabled:

Initially at boot, efi_memblock_x86_reserve_range() does early_memremap() of the EFI memory map passed as part of setup_data, as the following logs show:

...

[ 0.000000] efi: in efi_memblock_x86_reserve_range, phys map 0x27fff9110 
[ 0.000000] memblock_reserve: [0x000000027fff9110-0x000000027fffa12f] efi_memblock_x86_reserve_range+0x168/0x2a0

...

Later, efi_arch_mem_reserve() is invoked, which calls efi_memmap_alloc() which does memblock_phys_alloc() to insert new EFI memory descriptor into efi.memap:

...

[ 0.733263] memblock_reserve: [0x000000027ffcaf80-0x000000027ffcbfff] memblock_alloc_range_nid+0xf1/0x1b0 
[ 0.734787] efi: efi_arch_mem_reserve, efi phys map 0x27ffcaf80

...

Finally, at the end of boot, kexec_enter_virtual_mode() is called.

It does mapping of efi regions which were passed via setup_data.

So it unregisters the early mem-remapped EFI memmap and installs the new EFI memory map as below:

( Because of efi_arch_mem_reserve() getting invoked, the new EFI memmap phys base being remapped now is the memblock allocation done in efi_arch_mem_reserve()).

[ 4.042160] efi: efi memmap phys map 0x27ffcaf80

So, kexec_enter_virtual_mode() does the following :

	if (efi_memmap_init_late(efi.memmap.phys_map, <- refers to the new EFI memmap phys base allocated via memblock in efi_arch_mem_reserve().
	 	efi.memmap.desc_size * efi.memmap.nr_map)) { ...

This late init, does a memremap() on this memblock-allocated memory, but then immediately frees it :

drivers/firmware/efi/memmap.c:

int __init __efi_memmap_init(struct efi_memory_map_data *data) 
{

	..

	phys_map = data->phys_map; <- refers to the new EFI memmap phys base allocated via memblock in efi_arch_mem_reserve().

	if (data->flags & EFI_MEMMAP_LATE) 
		map.map = memremap(phys_map, data->size, MEMREMAP_WB);
	... 
	... 
	if (efi.memmap.flags & (EFI_MEMMAP_MEMBLOCK | EFI_MEMMAP_SLAB)) { 
		__efi_memmap_free(efi.memmap.phys_map, 
				efi.memmap.desc_size * efi.memmap.nr_map, efi.memmap.flags); 
	}

	...
	map.phys_map = data->phys_map;

	...

	efi.memmap = map;

	...

This happens as kexec_enter_virtual_mode() can only handle the early mapped EFI memmap and not the one which is memblock allocated by efi_arch_mem_reserve(). As seen above this memblock allocated (EFI_MEMMAP_MEMBLOCK tagged) memory gets freed.

This is confirmed by memblock debugging:

[ 4.044057] memblock_free_late: [0x000000027ffcaf80-0x000000027ffcbfff] __efi_memmap_free+0x66/0x80

So while this memory is memremapped, it has also been freed and then it gets into a use-after-free condition and subsequently gets corrupted.

This corruption is seen just before kexec-ing into the new kernel:

...
[   11.045522] PEFILE: Unsigned PE binary^M
[   11.060801] kexec-bzImage64: efi memmap phys map 0x27ffcaf80^M
...
[   11.061220] kexec-bzImage64: mmap entry, type = 11, va = 0xfffffffeffc00000, pa = 0xffc00000, np = 0x400, attr = 0x8000000000000001^M
[   11.061225] kexec-bzImage64: mmap entry, type = 6, va = 0xfffffffeffb04000, pa = 0x7f704000, np = 0x84, attr = 0x800000000000000f^M
[   11.061228] kexec-bzImage64: mmap entry, type = 4, va = 0xfffffffeff700000, pa = 0x7f100000, np = 0x300, attr = 0x0^M
[   11.061231] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M <- CORRUPTION!!!
[   11.061234] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061236] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061239] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061241] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061243] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061245] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061248] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061250] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061252] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061255] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061257] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061259] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061262] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061264] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061266] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061268] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061271] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061273] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061275] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061278] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061280] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061282] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061284] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061287] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061289] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061291] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061294] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061296] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061298] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061301] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061303] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061305] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061307] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061310] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061312] kexec-bzImage64: mmap entry, type = 0, va = 0x0, pa = 0x0, np = 0x0, attr = 0x0^M
[   11.061314] kexec-bzImage64: mmap entry, type = 14080, va = 0x14f29, pa = 0x36c0, np = 0x0, attr = 0x0^M
[   11.061317] kexec-bzImage64: mmap entry, type = 85808, va = 0x0, pa = 0x0, np = 0x72, attr = 0x14f40^M
[   11.061320] kexec-bzImage64: mmap entry, type = 0, va = 0x14f4b, pa = 0x65, np = 0x1, attr = 0x0^M
[   11.061323] kexec-bzImage64: mmap entry, type = 85840, va = 0x0, pa = 0x2, np = 0x69, attr = 0x14f59^M
[   11.061325] kexec-bzImage64: mmap entry, type = 0, va = 0x14f65, pa = 0x6c, np = 0x0, attr = 0x0^M
[   11.061328] kexec-bzImage64: mmap entry, type = 85871, va = 0x0, pa = 0x0, np = 0x7a, attr = 0x14f7f^M

...

This EFI phys map address 0x27ffcaf80 is being mem-remapped and also getting freed and then in use after free condition (while setting up the EFI memory map for the next kernel with kexec -s) in the above logs confirm the use-after-free case.

Looking at the above code flow, it makes sense to skip efi_arch_mem_reserve() to fix this issue, as it anyway needs to be skipped for kexec case.

Thanks, Ashish