[PATCH v5 14/21] arm64: head: create a temporary FDT mapping in the initial ID map

Ard Biesheuvel
2022-06-24T15:13:36Z

We need to access the DT very early to get at the command line and the
KASLR seed, which currently means we rely on some hacks to call into the
kernel before really calling into the kernel, which is undesirable.

So instead, let's create a mapping for the FDT in the initial ID map,
which is feasible now that it has been extended to cover more than a
single page or block, and can be updated in place to remap other output
addresses.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/arm64/include/asm/kernel-pgtable.h | 6 ++++--
arch/arm64/kernel/head.S | 14 +++++++++++++-
2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
index 5395e5a04f35..02e59fa8f293 100644
--- a/arch/arm64/include/asm/kernel-pgtable.h
+++ b/arch/arm64/include/asm/kernel-pgtable.h
@@ -8,6 +8,7 @@
#ifndef __ASM_KERNEL_PGTABLE_H
#define __ASM_KERNEL_PGTABLE_H

+#include <asm/boot.h>
#include <asm/pgtable-hwdef.h>
#include <asm/sparsemem.h>

@@ -88,10 +89,11 @@

/* the initial ID map may need two extra pages if it needs to be extended */
#if VA_BITS < 48
-#define INIT_IDMAP_DIR_SIZE (INIT_DIR_SIZE + (2 * PAGE_SIZE))
+#define INIT_IDMAP_DIR_SIZE ((INIT_IDMAP_DIR_PAGES + 2) * PAGE_SIZE)
#else
-#define INIT_IDMAP_DIR_SIZE INIT_DIR_SIZE
+#define INIT_IDMAP_DIR_SIZE (INIT_IDMAP_DIR_PAGES * PAGE_SIZE)
#endif
+#define INIT_IDMAP_DIR_PAGES EARLY_PAGES(KIMAGE_VADDR, _end + MAX_FDT_SIZE + SWAPPER_BLOCK_SIZE)

/* Initial memory map size */
#if ARM64_KERNEL_USES_PMD_MAPS
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index f1497f7b4da0..8283ff848328 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -83,6 +83,7 @@
*
* Register Scope Purpose
* x21 primary_entry() .. start_kernel() FDT pointer passed at boot in x0
+ * x22 create_idmap() .. start_kernel() ID map VA of the DT blob
* x23 primary_entry() .. start_kernel() physical misalignment/KASLR offset
* x28 clear_page_tables() callee preserved temp register
* x19/x20 __primary_switch() callee preserved temp registers
@@ -348,7 +349,7 @@ SYM_FUNC_START_LOCAL(create_idmap)
#endif
adrp x0, init_idmap_pg_dir
adrp x3, _text
- adrp x6, _end
+ adrp x6, _end + MAX_FDT_SIZE + SWAPPER_BLOCK_SIZE
mov x7, SWAPPER_RX_MMUFLAGS

map_memory x0, x1, x3, x6, x7, x3, IDMAP_PGD_ORDER, x10, x11, x12, x13, x14, EXTRA_SHIFT
@@ -362,6 +363,17 @@ SYM_FUNC_START_LOCAL(create_idmap)
mov x6, #SWAPPER_BLOCK_SHIFT
bl remap_region

+ /* Remap the FDT after the kernel image */
+ adrp x1, _text
+ adrp x22, _end + SWAPPER_BLOCK_SIZE
+ bic x2, x22, #SWAPPER_BLOCK_SIZE - 1
+ bfi x22, x21, #0, #SWAPPER_BLOCK_SHIFT // remapped FDT address
+ add x3, x2, #MAX_FDT_SIZE + SWAPPER_BLOCK_SIZE
+ bic x4, x21, #SWAPPER_BLOCK_SIZE - 1
+ mov x5, SWAPPER_RW_MMUFLAGS
+ mov x6, #SWAPPER_BLOCK_SHIFT
+ bl remap_region
+
/*
* Since the page tables have been populated with non-cacheable
* accesses (MMU disabled), invalidate those tables again to