Commit c7225494 authored Feb 18, 2020 by Ard Biesheuvel

efi/arm: Work around missing cache maintenance in decompressor handover



The EFI stub executes within the context of the zImage as it was
loaded by the firmware, which means it is treated as an ordinary
PE/COFF executable, which is loaded into memory, and cleaned to
the PoU to ensure that it can be executed safely while the MMU
and caches are on.

When the EFI stub hands over to the decompressor, we clean the caches
by set/way and disable the MMU and D-cache, to comply with the Linux
boot protocol for ARM. However, cache maintenance by set/way is not
sufficient to ensure that subsequent instruction fetches and data
accesses done with the MMU off see the correct data. This means that
proceeding as we do currently is not safe, especially since we also
perform data accesses with the MMU off, from a literal pool as well as
the stack.

So let's kick this can down the road a bit, and jump into the relocated
zImage before disabling the caches. This removes the requirement to
perform any by-VA cache maintenance on the original PE/COFF executable,
but it does require that the relocated zImage is cleaned to the PoC,
which is currently not the case. This will be addressed in a subsequent
patch.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

parent bb6d3fb3

arch/arm/boot/compressed/head.S

+14 −6

Original line number	Diff line number	Diff line
		@@ -1461,6 +1461,17 @@ ENTRY(efi_stub_entry)
		@ Preserve return value of efi_entry() in r4
		mov r4, r0
		bl cache_clean_flush

		@ The PE/COFF loader might not have cleaned the code we are
		@ running beyond the PoU, and so calling cache_off below from
		@ inside the PE/COFF loader allocated region is unsafe. Let's
		@ assume our own zImage relocation code did a better job, and
		@ jump into its version of this routine before proceeding.
		ldr r0, [sp] @ relocated zImage
		ldr r1, .Ljmp
		sub r1, r0, r1
		mov pc, r1 @ no mode switch
		0:
		bl cache_off

		@ Set parameters for booting zImage according to boot protocol
		@@ -1469,18 +1480,15 @@ ENTRY(efi_stub_entry)
		mov r0, #0
		mov r1, #0xFFFFFFFF
		mov r2, r4

		@ Branch to (possibly) relocated zImage that is in [sp]
		ldr lr, [sp]
		ldr ip, =start_offset
		add lr, lr, ip
		mov pc, lr @ no mode switch
		b __efi_start

		efi_load_fail:
		@ Return EFI_LOAD_ERROR to EFI firmware on error.
		ldr r0, =0x80000001
		ldmfd sp!, {ip, pc}
		ENDPROC(efi_stub_entry)
		.align 2
		.Ljmp: .long start - 0b
		#endif

		.align

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