arm64: always enable DEBUG_RODATA and remove the Kconfig option

Follow the example set by x86 in commit 9ccaf77c ("x86/mm:
Always enable CONFIG_DEBUG_RODATA and remove the Kconfig option"), and
make these protections a fundamental security feature rather than an
opt-in. This also results in a minor code simplification.

For those rare cases when users wish to disable this protection (e.g.
for debugging), this can be done by passing 'rodata=off' on the command
line.

As DEBUG_RODATA_ALIGN is only intended to address a performance/memory
tradeoff, and does not affect correctness, this is left user-selectable.
DEBUG_MODULE_RONX is also left user-selectable until the core code
provides a boot-time option to disable the protection for debugging
use-cases.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -122,6 +122,9 @@ config ARCH_PHYS_ADDR_T_64BIT
 config MMU
 	def_bool y
 
+config DEBUG_RODATA
+	def_bool y
+
 config ARM64_PAGE_SHIFT
 	int
 	default 16 if ARM64_64K_PAGES

arch/arm64/Kconfig.debug

@@ -49,16 +49,6 @@ config DEBUG_SET_MODULE_RONX
 
 	  If in doubt, say Y.
 
-config DEBUG_RODATA
-	bool "Make kernel text and rodata read-only"
-	default y
-	help
-	  If this is set, kernel text and rodata will be made read-only. This
-	  is to help catch accidental or malicious attempts to change the
-	  kernel's executable code.
-
-	  If in doubt, say Y.
-
 config DEBUG_ALIGN_RODATA
 	depends on DEBUG_RODATA
 	bool "Align linker sections up to SECTION_SIZE"

arch/arm64/kernel/insn.c

@@ -96,7 +96,7 @@ static void __kprobes *patch_map(void *addr, int fixmap)
 
 	if (module && IS_ENABLED(CONFIG_DEBUG_SET_MODULE_RONX))
 		page = vmalloc_to_page(addr);
-	else if (!module && IS_ENABLED(CONFIG_DEBUG_RODATA))
+	else if (!module)
 		page = pfn_to_page(PHYS_PFN(__pa(addr)));
 	else
 		return addr;