[PATCH] arm64: align randomized TEXT_OFFSET on 4 kB boundary

[Ard Biesheuvel]

When booting via UEFI, the kernel Image is loaded at a 4 kB boundary and
the embedded EFI stub is executed in place. The EFI stub relocates the
Image to reside TEXT_OFFSET bytes above a 2 MB boundary, and jumps into
the kernel proper.

In AArch64, PC relative symbol references are emitted using adrp/add or
adrp/ldr pairs, where the offset into a 4 kB page is resolved using a
separate :lo12: relocation. This implicitly assumes that the code will
always be executed at the same relative offset with respect to a 4 kB
boundary, or the references will point to the wrong address.

This means we should link the kernel at a 4 kB aligned base address in
order to remain compatible with the base address the UEFI loader uses
when doing the initial load of Image. So update the code that generates
TEXT_OFFSET to choose a multiple of 4 kB.

At the same time, update the code so it chooses from the interval [0..2MB)
as the author originally intended.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 arch/arm64/Makefile      | 2 +-
 arch/arm64/kernel/head.S | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 57833546bf00..2df5e5daeebe 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -39,7 +39,7 @@ head-y		:= arch/arm64/kernel/head.o
 
 # The byte offset of the kernel image in RAM from the start of RAM.
 ifeq ($(CONFIG_ARM64_RANDOMIZE_TEXT_OFFSET), y)
-TEXT_OFFSET := $(shell awk 'BEGIN {srand(); printf "0x%04x0\n", int(65535 * rand())}')
+TEXT_OFFSET := $(shell awk 'BEGIN {srand(); printf "0x%03x000\n", int(512 * rand())}')
 else
 TEXT_OFFSET := 0x00080000
 endif
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index b6ca95aee348..ae055a18b04a 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -38,11 +38,11 @@
 
 #define KERNEL_RAM_VADDR	(PAGE_OFFSET + TEXT_OFFSET)
 
-#if (TEXT_OFFSET & 0xf) != 0
-#error TEXT_OFFSET must be at least 16B aligned
-#elif (PAGE_OFFSET & 0xfffff) != 0
+#if (TEXT_OFFSET & 0xfff) != 0
+#error TEXT_OFFSET must be at least 4kB aligned
+#elif (PAGE_OFFSET & 0x1fffff) != 0
 #error PAGE_OFFSET must be at least 2MB aligned
-#elif TEXT_OFFSET > 0xfffff
+#elif TEXT_OFFSET > 0x1fffff
 #error TEXT_OFFSET must be less than 2MB
 #endif
 
-- 
1.8.3.2