[PATCH v2 09/10] arm64: mm: Map the kernel data/bss read-only in the linear map

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

On systems where the bootloader adheres to the original arm64 boot
protocol, the placement of the kernel in the physical address space is
highly predictable, and this makes the placement of its linear alias in
the kernel virtual address space equally predictable, given the lack of
randomization of the linear map.

The linear aliases of the kernel text and rodata regions are already
mapped read-only, but the kernel data and bss are mapped read-write in
this region. This is not needed, so map them read-only as well.

Note that the statically allocated kernel page tables do need to be
modifiable via the linear map, so leave these mapped read-write.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/include/asm/sections.h |  1 +
 arch/arm64/mm/mmu.c               | 10 ++++++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h
index 51b0d594239e..f7fe2bcbfd03 100644
--- a/arch/arm64/include/asm/sections.h
+++ b/arch/arm64/include/asm/sections.h
@@ -23,6 +23,7 @@ extern char __irqentry_text_start[], __irqentry_text_end[];
 extern char __mmuoff_data_start[], __mmuoff_data_end[];
 extern char __entry_tramp_text_start[], __entry_tramp_text_end[];
 extern char __relocate_new_kernel_start[], __relocate_new_kernel_end[];
+extern char __pgdir_start[];
 
 static inline size_t entry_tramp_text_size(void)
 {
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 18415d4743bf..fdbbb018adc5 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1122,7 +1122,9 @@ static void __init map_mem(void)
 {
 	static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
 	phys_addr_t kernel_start = __pa_symbol(_text);
-	phys_addr_t kernel_end = __pa_symbol(__init_begin);
+	phys_addr_t init_begin = __pa_symbol(__init_begin);
+	phys_addr_t init_end = __pa_symbol(__init_end);
+	phys_addr_t kernel_end = __pa_symbol(__pgdir_start);
 	phys_addr_t start, end;
 	phys_addr_t early_kfence_pool;
 	int flags = NO_EXEC_MAPPINGS;
@@ -1158,7 +1160,9 @@ static void __init map_mem(void)
 	 * Note that contiguous mappings cannot be remapped in this way,
 	 * so we should avoid them here.
 	 */
-	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL,
+	__map_memblock(kernel_start, init_begin, PAGE_KERNEL,
+		       flags | NO_CONT_MAPPINGS);
+	__map_memblock(init_end, kernel_end, PAGE_KERNEL,
 		       flags | NO_CONT_MAPPINGS);
 
 	/* map all the memory banks */
@@ -1172,6 +1176,8 @@ static void __init map_mem(void)
 			       flags);
 	}
 
+	__map_memblock(init_end, kernel_end, PAGE_KERNEL_RO,
+		       flags | NO_CONT_MAPPINGS);
 	arm64_kfence_map_pool(early_kfence_pool);
 }
 
-- 
2.52.0.457.g6b5491de43-goog