efi/arm: Clean EFI stub exit code from cache instead of avoiding it

The following commit:

  c7225494 ("efi/arm: Work around missing cache maintenance in decompressor handover")

modified the EFI handover code written in assembler to work around the
missing cache maintenance of the piece of code that is executed after the
MMU and caches are turned off.

Due to the fact that this sequence incorporates a subroutine call, cleaning
that code from the cache is not a matter of simply passing the start and end of
the currently running subroutine into cache_clean_flush(), which is why
instead, the code jumps across into the cleaned copy of the image.

However, this assumes that this copy is executable, and this means we
expect EFI_LOADER_DATA regions to be executable as well, which is not
a reasonable assumption to make, even if this is true for most UEFI
implementations today.

So change this back, and add a cache_clean_flush() call to cover the
remaining code in the subroutine, and any code it may execute in the
context of cache_off().
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-efi@vger.kernel.org
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20200228121408.9075-5-ardb@kernel.org

arch/arm/boot/compressed/head.S

@@ -1452,13 +1452,11 @@ ENTRY(efi_enter_kernel)
 
 		@ 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	r1, .Ljmp
-		sub	r1, r7, r1
-		mov	pc, r1				@ no mode switch
-0:
+		@ inside the PE/COFF loader allocated region is unsafe unless
+		@ we explicitly clean it to the PoC.
+		adr	r0, call_cache_fn		@ region of code we will
+		adr	r1, 0f				@ run with MMU off
+		bl	cache_clean_flush
 		bl	cache_off
 
 		@ Set parameters for booting zImage according to boot protocol
@@ -1467,10 +1465,10 @@ ENTRY(efi_enter_kernel)
 		mov	r0, #0
 		mov	r1, #0xFFFFFFFF
 		mov	r2, r4
-		b	__efi_start
+		add	r7, r7, #(__efi_start - start)
+		mov	pc, r7				@ no mode switch
 ENDPROC(efi_enter_kernel)
-		.align	2
-.Ljmp:		.long	start - 0b
+0:
 #endif
 
 		.align