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>

diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index 1c1b7562984292107a0aca17481feba7fae330d8..4ee8e90b7a45eb8ed7babf5418ab9cf92b2ae9f2 100644 (file)
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -28,4 +28,19 @@ config PID_IN_CONTEXTIDR
          instructions during context switch. Say Y here only if you are
          planning to use hardware trace tools with this kernel.
 
+config ARM64_RANDOMIZE_TEXT_OFFSET
+       bool "Randomize TEXT_OFFSET at build time"
+       help
+         Say Y here if you want the image load offset (AKA TEXT_OFFSET)
+         of the kernel to be randomized at build-time. When selected,
+         this option will cause TEXT_OFFSET to be randomized upon any
+         build of the kernel, and the offset will be reflected in the
+         text_offset field of the resulting Image. This can be used to
+         fuzz-test bootloaders which respect text_offset.
+
+         This option is intended for bootloader and/or kernel testing
+         only. Bootloaders must make no assumptions regarding the value
+         of TEXT_OFFSET and platforms must not require a specific
+         value.
+
 endmenu

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 8185a913c5ed5ab2d659c119903666409bbb1793..e8d025c1459e034ddaa11efaf11e8da31808e365 100644 (file)
--- 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
 

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 3ba0fc02c0decb60d5eddd27abf3d298dab54a81..69dafe9621fda4ec1482587a521c0cf5fe8ecf5b 100644 (file)
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -37,8 +37,12 @@
 
 #define KERNEL_RAM_VADDR       (PAGE_OFFSET + TEXT_OFFSET)
 
-#if (KERNEL_RAM_VADDR & 0xfffff) != 0x80000
-#error KERNEL_RAM_VADDR must start at 0xXXX80000
+#if (TEXT_OFFSET & 0xf) != 0
+#error TEXT_OFFSET must be at least 16B aligned
+#elif (PAGE_OFFSET & 0xfffff) != 0
+#error PAGE_OFFSET must be at least 2MB aligned
+#elif TEXT_OFFSET > 0xfffff
+#error TEXT_OFFSET must be less than 2MB
 #endif
 
        .macro  pgtbl, ttb0, ttb1, virt_to_phys

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index a814768ae1485ff1dd460a3f1c50298dc416c3f6..97f0c0429dfa647fc9e07eb349768b4ebc750e71 100644 (file)
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -125,3 +125,8 @@ SECTIONS
  */
 ASSERT(((__hyp_idmap_text_start + PAGE_SIZE) > __hyp_idmap_text_end),
        "HYP init code too big")
+
+/*
+ * If padding is applied before .head.text, virt<->phys conversions will fail.
+ */
+ASSERT(_text == (PAGE_OFFSET + TEXT_OFFSET), "HEAD is misaligned")