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>

diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 088b0a0608761d5eff23ff5bfc3dac674bbb6610..39f7071d47c78e781c433c395cb0d22b85b451d0 100644 (file)
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -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