arm64: fix R/O permissions of FDT mapping

The mapping permissions of the FDT are set to 'PAGE_KERNEL | PTE_RDONLY'
in an attempt to map the FDT as read-only. However, not only does this
break at build time under STRICT_MM_TYPECHECKS (since the two terms are
of different types in that case), it also results in both the PTE_WRITE
and PTE_RDONLY attributes to be set, which means the region is still
writable under ARMv8.1 DBM (and an attempted write will simply clear the
PT_RDONLY bit).

So instead, define PAGE_KERNEL_RO (which already has an established
meaning across architectures) and use that instead.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

2 files changed, 2 insertions, 1 deletions

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index c3d22a507648..1c99d5610bc7 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -78,6 +78,7 @@ extern void __pgd_error(const char *file, int line, unsigned long val);
 #define _PAGE_DEFAULT           (PROT_DEFAULT | PTE_ATTRINDX(MT_NORMAL))
 
 #define PAGE_KERNEL             __pgprot(_PAGE_DEFAULT | PTE_PXN | PTE_UXN | PTE_DIRTY | PTE_WRITE)
+#define PAGE_KERNEL_RO          __pgprot(_PAGE_DEFAULT | PTE_PXN | PTE_UXN | PTE_DIRTY | PTE_RDONLY)
 #define PAGE_KERNEL_EXEC        __pgprot(_PAGE_DEFAULT | PTE_UXN | PTE_DIRTY | PTE_WRITE)
 #define PAGE_KERNEL_EXEC_CONT   __pgprot(_PAGE_DEFAULT | PTE_UXN | PTE_DIRTY | PTE_WRITE | PTE_CONT)
 
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 2784f052df5e..5e23de53b747 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -691,7 +691,7 @@ void __set_fixmap(enum fixed_addresses idx,
 void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
 {
         const u64 dt_virt_base = __fix_to_virt(FIX_FDT);
-        pgprot_t prot = PAGE_KERNEL | PTE_RDONLY;
+        pgprot_t prot = PAGE_KERNEL_RO;
         int size, offset;
         void *dt_virt;