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@xxxxxxxxxx> --- arch/arm/boot/compressed/head.S | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S index 088b0a060876..4010f6e4b047 100644 --- a/arch/arm/boot/compressed/head.S +++ b/arch/arm/boot/compressed/head.S @@ -1458,6 +1458,18 @@ ENTRY(efi_stub_entry) cmn r0, #1 beq efi_load_fail + @ 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 it before proceeding. + ldr lr, [sp] @ relocated zImage + ldr ip, 0f + sub lr, lr, ip + mov pc, lr @ no mode switch + .align 2 +0: .long start - (. + 4) + @ Preserve return value of efi_entry() in r4 mov r4, r0 bl cache_clean_flush @@ -1469,12 +1481,7 @@ 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. -- 2.17.1
