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Date:   Mon, 13 Jun 2022 16:45:46 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     linux-hardening@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: [PATCH v4 22/26] arm64: mm: move ro_after_init section into the data segment

Currently, the ro_after_init sections sits right in the middle of the
text/rodata/inittext segment, making it difficult to map any of those
non-writable during early boot. So instead, move it to the start of
.data, and update the init sequences so that the section is remapped
read-only once startup completes.

Note that this moves the entire HYP data section into .data as well -
this likely needs to remain as a single block for now, but could perhaps
split into a .rodata and .data..ro_after_init section later.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/arm64/kernel/vmlinux.lds.S | 42 ++++++++++++--------
 arch/arm64/mm/mmu.c             | 29 ++++++++------
 2 files changed, 42 insertions(+), 29 deletions(-)

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 45131e354e27..736aca63dad1 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -59,6 +59,7 @@
 
 #define RO_EXCEPTION_TABLE_ALIGN	4
 #define RUNTIME_DISCARD_EXIT
+#define RO_AFTER_INIT_DATA
 
 #include <asm-generic/vmlinux.lds.h>
 #include <asm/cache.h>
@@ -188,30 +189,13 @@ SECTIONS
 	/* everything from this point to __init_begin will be marked RO NX */
 	RO_DATA(PAGE_SIZE)
 
-	HYPERVISOR_DATA_SECTIONS
-
 	/* code sections that are never executed via the kernel mapping */
 	.rodata.text : {
 		TRAMP_TEXT
 		HIBERNATE_TEXT
 		KEXEC_TEXT
-		. = ALIGN(PAGE_SIZE);
 	}
 
-	idmap_pg_dir = .;
-	. += PAGE_SIZE;
-
-#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
-	tramp_pg_dir = .;
-	. += PAGE_SIZE;
-#endif
-
-	reserved_pg_dir = .;
-	. += PAGE_SIZE;
-
-	swapper_pg_dir = .;
-	. += PAGE_SIZE;
-
 	. = ALIGN(SEGMENT_ALIGN);
 	__init_begin = .;
 	__inittext_begin = .;
@@ -274,6 +258,30 @@ SECTIONS
 
 	_data = .;
 	_sdata = .;
+
+	__start_ro_after_init = .;
+	idmap_pg_dir = .;
+	. += PAGE_SIZE;
+
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+	tramp_pg_dir = .;
+	. += PAGE_SIZE;
+#endif
+	reserved_pg_dir = .;
+	. += PAGE_SIZE;
+
+	swapper_pg_dir = .;
+	. += PAGE_SIZE;
+
+	HYPERVISOR_DATA_SECTIONS
+
+	.data.ro_after_init : {
+		*(.data..ro_after_init)
+		JUMP_TABLE_DATA
+		. = ALIGN(SEGMENT_ALIGN);
+		__end_ro_after_init = .;
+	}
+
 	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
 
 	/*
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9828ad826837..e9b074ffc768 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -495,11 +495,17 @@ static void __init __map_memblock(pgd_t *pgdp, phys_addr_t start,
 void __init mark_linear_text_alias_ro(void)
 {
 	/*
-	 * Remove the write permissions from the linear alias of .text/.rodata
+	 * Remove the write permissions from the linear alias of .text/.rodata/ro_after_init
 	 */
 	update_mapping_prot(__pa_symbol(_stext), (unsigned long)lm_alias(_stext),
 			    (unsigned long)__init_begin - (unsigned long)_stext,
 			    PAGE_KERNEL_RO);
+
+	update_mapping_prot(__pa_symbol(__start_ro_after_init),
+			    (unsigned long)lm_alias(__start_ro_after_init),
+			    (unsigned long)__end_ro_after_init -
+			    (unsigned long)__start_ro_after_init,
+			    PAGE_KERNEL_RO);
 }
 
 static bool crash_mem_map __initdata;
@@ -608,12 +614,10 @@ void mark_rodata_ro(void)
 {
 	unsigned long section_size;
 
-	/*
-	 * mark .rodata as read only. Use __init_begin rather than __end_rodata
-	 * to cover NOTES and EXCEPTION_TABLE.
-	 */
-	section_size = (unsigned long)__init_begin - (unsigned long)__start_rodata;
-	update_mapping_prot(__pa_symbol(__start_rodata), (unsigned long)__start_rodata,
+	section_size = (unsigned long)__end_ro_after_init -
+		       (unsigned long)__start_ro_after_init;
+	update_mapping_prot(__pa_symbol(__start_ro_after_init),
+			    (unsigned long)__start_ro_after_init,
 			    section_size, PAGE_KERNEL_RO);
 
 	debug_checkwx();
@@ -733,18 +737,19 @@ static void __init map_kernel(pgd_t *pgdp)
 		text_prot = __pgprot_modify(text_prot, PTE_GP, PTE_GP);
 
 	/*
-	 * Only rodata will be remapped with different permissions later on,
-	 * all other segments are allowed to use contiguous mappings.
+	 * Only data will be partially remapped with different permissions
+	 * later on, all other segments are allowed to use contiguous mappings.
 	 */
 	map_kernel_segment(pgdp, _stext, _etext, text_prot, &vmlinux_text, 0,
 			   VM_NO_GUARD);
-	map_kernel_segment(pgdp, __start_rodata, __inittext_begin, PAGE_KERNEL,
-			   &vmlinux_rodata, NO_CONT_MAPPINGS, VM_NO_GUARD);
+	map_kernel_segment(pgdp, __start_rodata, __inittext_begin, PAGE_KERNEL_RO,
+			   &vmlinux_rodata, 0, VM_NO_GUARD);
 	map_kernel_segment(pgdp, __inittext_begin, __inittext_end, text_prot,
 			   &vmlinux_inittext, 0, VM_NO_GUARD);
 	map_kernel_segment(pgdp, __initdata_begin, __initdata_end, PAGE_KERNEL,
 			   &vmlinux_initdata, 0, VM_NO_GUARD);
-	map_kernel_segment(pgdp, _data, _end, PAGE_KERNEL, &vmlinux_data, 0, 0);
+	map_kernel_segment(pgdp, _data, _end, PAGE_KERNEL, &vmlinux_data,
+			   NO_CONT_MAPPINGS | NO_BLOCK_MAPPINGS, 0);
 
 	if (!READ_ONCE(pgd_val(*pgd_offset_pgd(pgdp, FIXADDR_START)))) {
 		/*
-- 
2.30.2

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