arm64: Enable TEXT_OFFSET fuzzing

The arm64 Image header contains a text_offset field which bootloaders
are supposed to read to determine the offset (from a 2MB aligned "start
of memory" per booting.txt) at which to load the kernel. The offset is
not well respected by bootloaders at present, and due to the lack of
variation there is little incentive to support it. This is unfortunate
for the sake of future kernels where we may wish to vary the text offset
(even zeroing it).

This patch adds options to arm64 to enable fuzz-testing of text_offset.
CONFIG_ARM64_RANDOMIZE_TEXT_OFFSET forces the text offset to a random
16-byte aligned value value in the range [0..2MB) upon a build of the
kernel. It is recommended that distribution kernels enable randomization
to test bootloaders such that any compliance issues can be fixed early.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 8185a913c5ed..e8d025c1459e 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -38,7 +38,11 @@ CHECKFLAGS	+= -D__aarch64__
 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())}')
+else
 TEXT_OFFSET := 0x00080000
+endif
 
 export	TEXT_OFFSET GZFLAGS