Subject: [PATCHv4 7/7] arm64: add better page protections to arm64

From: [email-masked] (Laura Abbott)
Date: Mon, 27 Oct 2014 13:12:32 -0700

Add page protections for arm64 similar to those in arm.
This is for security reasons to prevent certain classes
of exploits. The current method:

- Map all memory as either RWX or RW. We round to the nearest
  section to avoid creating page tables before everything is mapped
- Once everything is mapped, if either end of the RWX section should
  not be X, we split the PMD and remap as necessary
- When initmem is to be freed, we change the permissions back to
  RW (using stop machine if necessary to flush the TLB)
- If CONFIG_DEBUG_RODATA is set, the read only sections are set
  read only.

Signed-off-by: Laura Abbott <[email-masked]>
---
v4: Combined the Kconfig options
---
 arch/arm64/Kconfig.debug            |  23 +++
 arch/arm64/include/asm/cacheflush.h |   4 +
 arch/arm64/kernel/vmlinux.lds.S     |  17 ++
 arch/arm64/mm/init.c                |   1 +
 arch/arm64/mm/mm.h                  |   2 +
 arch/arm64/mm/mmu.c                 | 303 +++++++++++++++++++++++++++++++-----
 6 files changed, 314 insertions(+), 36 deletions(-)

diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index 0a12933..1b25015 100644
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -54,4 +54,27 @@ config DEBUG_SET_MODULE_RONX
           against certain classes of kernel exploits.
           If in doubt, say "N".
 
+config DEBUG_RODATA
+	bool "Make kernel text and rodata read-only"
+	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. Additionally splits rodata from kernel
+	  text so it can be made explicitly non-executable.
+
+          If in doubt, say Y
+
+config DEBUG_ALIGN_RODATA
+	depends on DEBUG_RODATA
+	bool "Align linker sections up to SECTION_SIZE"
+	help
+	  If this option is enabled, sections that may potentially be marked as
+	  read only or non-executable will be aligned up to the section size of
+	  the kernel. This prevents sections from being split into pages and
+	  avoids a potential TLB penalty. The downside is an increase in
+	  alignment and potentially wasted space. Turn on this option if
+	  performance is more important than memory pressure.
+
+	  If in doubt, say N
+
 endmenu
diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
index 689b637..0014207 100644
--- a/arch/arm64/include/asm/cacheflush.h
+++ b/arch/arm64/include/asm/cacheflush.h
@@ -152,4 +152,8 @@ int set_memory_ro(unsigned long addr, int numpages);
 int set_memory_rw(unsigned long addr, int numpages);
 int set_memory_x(unsigned long addr, int numpages);
 int set_memory_nx(unsigned long addr, int numpages);
+
+#ifdef CONFIG_DEBUG_RODATA
+void mark_rodata_ro(void);
+#endif
 #endif
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index edf8715..83424ad 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -8,6 +8,7 @@
 #include <asm/thread_info.h>
 #include <asm/memory.h>
 #include <asm/page.h>
+#include <asm/pgtable.h>
 
 #include "image.h"
 
@@ -54,6 +55,9 @@ SECTIONS
 		_text = .;
 		HEAD_TEXT
 	}
+#ifdef DEBUG_ALIGN_RODATA
+	. = ALIGN(1<<SECTION_SHIFT);
+#endif
 	.text : {			/* Real text segment		*/
 		_stext = .;		/* Text and read-only data	*/
 			__exception_text_start = .;
@@ -70,19 +74,32 @@ SECTIONS
 		*(.got)			/* Global offset table		*/
 	}
 
+#ifdef DEBUG_ALIGN_RODATA
+	. = ALIGN(1<<SECTION_SHIFT);
+#endif
 	RO_DATA(PAGE_SIZE)
 	EXCEPTION_TABLE(8)
 	NOTES
 	_etext = .;			/* End of text and rodata section */
 
+#ifdef DEBUG_ALIGN_RODATA
+	. = ALIGN(1<<SECTION_SHIFT);
+#else
 	. = ALIGN(PAGE_SIZE);
+#endif
 	__init_begin = .;
 
 	INIT_TEXT_SECTION(8)
 	.exit.text : {
 		ARM_EXIT_KEEP(EXIT_TEXT)
 	}
+
+#ifdef DEBUG_ALIGN_RODATA
+	. = ALIGN(1<<SECTION_SHIFT);
+	__init_data_begin = .;
+#else
 	. = ALIGN(16);
+#endif
 	.init.data : {
 		INIT_DATA
 		INIT_SETUP(16)
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 494297c..61f44c7 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -324,6 +324,7 @@ void __init mem_init(void)
 
 void free_initmem(void)
 {
+	fixup_init();
 	free_initmem_default(0);
 }
 
diff --git a/arch/arm64/mm/mm.h b/arch/arm64/mm/mm.h
index d519f4f..82347d8 100644
--- a/arch/arm64/mm/mm.h
+++ b/arch/arm64/mm/mm.h
@@ -1,2 +1,4 @@
 extern void __init bootmem_init(void);
 extern void __init arm64_swiotlb_init(void);
+
+void fixup_init(void);
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index e92f633..0428294 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -26,6 +26,7 @@
 #include <linux/memblock.h>
 #include <linux/fs.h>
 #include <linux/io.h>
+#include <linux/stop_machine.h>
 
 #include <asm/cputype.h>
 #include <asm/fixmap.h>
@@ -137,17 +138,55 @@ static void __init *early_alloc(unsigned long sz)
 	return ptr;
 }
 
-static void __init alloc_init_pte(pmd_t *pmd, unsigned long addr,
+/*
+ * remap a PMD into pages
+ */
+static noinline void __ref split_pmd(pmd_t *pmd, bool early)
+{
+	pte_t *pte, *start_pte;
+	unsigned long pfn;
+	int i = 0;
+
+	if (early)
+		start_pte = pte = early_alloc(PTRS_PER_PTE*sizeof(pte_t));
+	else
+		start_pte = pte = (pte_t *)__get_free_page(PGALLOC_GFP);
+
+	BUG_ON(!pte);
+
+	pfn = pmd_pfn(*pmd);
+
+	do {
+		/*
+		 * Need to have the least restrictive permissions available
+		 * permissions will be fixed up later
+		 */
+		set_pte(pte, pfn_pte(pfn, PAGE_KERNEL_EXEC));
+		pfn++;
+	} while (pte++, i++, i < PTRS_PER_PTE);
+
+
+	__pmd_populate(pmd, __pa(start_pte), PMD_TYPE_TABLE);
+	flush_tlb_all();
+}
+
+static void __ref alloc_init_pte(pmd_t *pmd, unsigned long addr,
 				  unsigned long end, unsigned long pfn,
-				  pgprot_t prot)
+				  pgprot_t prot, bool early)
 {
 	pte_t *pte;
 
 	if (pmd_none(*pmd)) {
-		pte = early_alloc(PTRS_PER_PTE * sizeof(pte_t));
+		if (early)
+			pte = early_alloc(PTRS_PER_PTE * sizeof(pte_t));
+		else
+			pte = (pte_t *)__get_free_page(PGALLOC_GFP);
+		BUG_ON(!pte);
 		__pmd_populate(pmd, __pa(pte), PMD_TYPE_TABLE);
 	}
-	BUG_ON(pmd_bad(*pmd));
+
+	if (pmd_bad(*pmd))
+		split_pmd(pmd, early);
 
 	pte = pte_offset_kernel(pmd, addr);
 	do {
@@ -156,29 +195,52 @@ static void __init alloc_init_pte(pmd_t *pmd, unsigned long addr,
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 }
 
-static void __init alloc_init_pmd(pud_t *pud, unsigned long addr,
+void __ref split_pud(pud_t *old_pud, pmd_t *pmd)
+{
+	unsigned long pfn = pud_pfn(*old_pud);
+        const pmdval_t mask = PMD_SECT_USER | PMD_SECT_PXN | PMD_SECT_UXN |
+                              PMD_SECT_RDONLY | PMD_SECT_PROT_NONE |
+                              PMD_SECT_VALID;
+	pgprot_t prot = __pgprot(pud_val(*old_pud) & mask);
+	int i = 0;
+
+	do {
+		set_pmd(pmd, pfn_pmd(pfn, prot));
+		pfn++;
+	} while (pmd++, i++, i < PTRS_PER_PMD);
+}
+
+static void __ref alloc_init_pmd(pud_t *pud, unsigned long addr,
 				  unsigned long end, phys_addr_t phys,
-				  int map_io)
+				  pgprot_t sect_prot, pgprot_t pte_prot,
+				  bool early, int map_io)
 {
 	pmd_t *pmd;
 	unsigned long next;
-	pmdval_t prot_sect;
-	pgprot_t prot_pte;
 
 	if (map_io) {
-		prot_sect = PROT_SECT_DEVICE_nGnRE;
-		prot_pte = __pgprot(PROT_DEVICE_nGnRE);
-	} else {
-		prot_sect = PROT_SECT_NORMAL_EXEC;
-		prot_pte = PAGE_KERNEL_EXEC;
+		sect_prot = PROT_SECT_DEVICE_nGnRE;
+		pte_prot = __pgprot(PROT_DEVICE_nGnRE);
 	}
 
 	/*
 	 * Check for initial section mappings in the pgd/pud and remove them.
 	 */
 	if (pud_none(*pud) || pud_bad(*pud)) {
-		pmd = early_alloc(PTRS_PER_PMD * sizeof(pmd_t));
+		if (early)
+			pmd = early_alloc(PTRS_PER_PMD * sizeof(pmd_t));
+		else
+			pmd = pmd_alloc_one(&init_mm, addr);
+		BUG_ON(!pmd);
+		if (pud_sect(*pud)) {
+			/*
+			 * need to have the 1G of mappings continue to be
+			 * present
+			 */
+			split_pud(pud, pmd);
+		}
 		pud_populate(&init_mm, pud, pmd);
+		flush_tlb_all();
 	}
 
 	pmd = pmd_offset(pud, addr);
@@ -186,8 +248,8 @@ static void __init alloc_init_pmd(pud_t *pud, unsigned long addr,
 		next = pmd_addr_end(addr, end);
 		/* try section mapping first */
 		if (((addr | next | phys) & ~SECTION_MASK) == 0) {
-			pmd_t old_pmd =*pmd;
-			set_pmd(pmd, __pmd(phys | prot_sect));
+			pmd_t old_pmd = *pmd;
+			set_pmd(pmd, __pmd(phys | sect_prot));
 			/*
 			 * Check for previous table entries created during
 			 * boot (__create_page_tables) and flush them.
@@ -196,15 +258,16 @@ static void __init alloc_init_pmd(pud_t *pud, unsigned long addr,
 				flush_tlb_all();
 		} else {
 			alloc_init_pte(pmd, addr, next, __phys_to_pfn(phys),
-				       prot_pte);
+					pte_prot, early);
 		}
 		phys += next - addr;
 	} while (pmd++, addr = next, addr != end);
 }
 
-static void __init alloc_init_pud(pgd_t *pgd, unsigned long addr,
+static void __ref alloc_init_pud(pgd_t *pgd, unsigned long addr,
 				  unsigned long end, unsigned long phys,
-				  int map_io)
+				  pgprot_t sect_prot, pgprot_t pte_prot,
+				  bool early, int map_io)
 {
 	pud_t *pud;
 	unsigned long next;
@@ -222,10 +285,10 @@ static void __init alloc_init_pud(pgd_t *pgd, unsigned long addr,
 		/*
 		 * For 4K granule only, attempt to put down a 1GB block
 		 */
-		if (!map_io && (PAGE_SHIFT == 12) &&
+		if (!map_io && early && (PAGE_SHIFT == 12) &&
 		    ((addr | next | phys) & ~PUD_MASK) == 0) {
 			pud_t old_pud = *pud;
-			set_pud(pud, __pud(phys | PROT_SECT_NORMAL_EXEC));
+			set_pud(pud, __pud(phys | sect_prot));
 
 			/*
 			 * If we have an old value for a pud, it will
@@ -240,7 +303,8 @@ static void __init alloc_init_pud(pgd_t *pgd, unsigned long addr,
 				flush_tlb_all();
 			}
 		} else {
-			alloc_init_pmd(pud, addr, next, phys, map_io);
+			alloc_init_pmd(pud, addr, next, phys, sect_prot,
+					pte_prot, early, map_io);
 		}
 		phys += next - addr;
 	} while (pud++, addr = next, addr != end);
@@ -250,9 +314,11 @@ static void __init alloc_init_pud(pgd_t *pgd, unsigned long addr,
  * Create the page directory entries and any necessary page tables for the
  * mapping specified by 'md'.
  */
-static void __init __create_mapping(pgd_t *pgd, phys_addr_t phys,
-				    unsigned long virt, phys_addr_t size,
-				    int map_io)
+static void __ref __create_mapping(pgd_t *pgd, phys_addr_t phys,
+				  unsigned long virt,
+				  phys_addr_t size,
+				  pgprot_t sect_prot, pgprot_t pte_prot,
+				  int map_io, bool early)
 {
 	unsigned long addr, length, end, next;
 
@@ -262,31 +328,140 @@ static void __init __create_mapping(pgd_t *pgd, phys_addr_t phys,
 	end = addr + length;
 	do {
 		next = pgd_addr_end(addr, end);
-		alloc_init_pud(pgd, addr, next, phys, map_io);
+		alloc_init_pud(pgd, addr, next, phys, sect_prot, pte_prot,
+				early, map_io);
 		phys += next - addr;
 	} while (pgd++, addr = next, addr != end);
 }
 
-static void __init create_mapping(phys_addr_t phys, unsigned long virt,
-				  phys_addr_t size)
+void __init create_id_mapping(phys_addr_t addr, phys_addr_t size, int map_io)
+{
+	if ((addr >> PGDIR_SHIFT) >= ARRAY_SIZE(idmap_pg_dir)) {
+		pr_warn("BUG: not creating id mapping for %pa\n", &addr);
+		return;
+	}
+	__create_mapping(&idmap_pg_dir[pgd_index(addr)],
+			 addr, addr, size, PROT_SECT_NORMAL_EXEC,
+			 PAGE_KERNEL_EXEC, map_io, false);
+}
+
+static inline pmd_t *pmd_off_k(unsigned long virt)
+{
+	return pmd_offset(pud_offset(pgd_offset_k(virt), virt), virt);
+}
+
+#ifdef CONFIG_EARLY_PRINTK
+/*
+ * Create an early I/O mapping using the pgd/pmd entries already populated
+ * in head.S as this function is called too early to allocated any memory. The
+ * mapping size is 2MB with 4KB pages or 64KB or 64KB pages.
+ */
+void __iomem * __init early_io_map(phys_addr_t phys, unsigned long virt)
+{
+	unsigned long size, mask;
+	bool page64k = IS_ENABLED(CONFIG_ARM64_64K_PAGES);
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+
+	/*
+	 * No early pte entries with !ARM64_64K_PAGES configuration, so using
+	 * sections (pmd).
+	 */
+	size = page64k ? PAGE_SIZE : SECTION_SIZE;
+	mask = ~(size - 1);
+
+	pgd = pgd_offset_k(virt);
+	pud = pud_offset(pgd, virt);
+	if (pud_none(*pud))
+		return NULL;
+	pmd = pmd_offset(pud, virt);
+
+	if (page64k) {
+		if (pmd_none(*pmd))
+			return NULL;
+		pte = pte_offset_kernel(pmd, virt);
+		set_pte(pte, __pte((phys & mask) | PROT_DEVICE_nGnRE));
+	} else {
+		set_pmd(pmd, __pmd((phys & mask) | PROT_SECT_DEVICE_nGnRE));
+	}
+
+	return (void __iomem *)((virt & mask) + (phys & ~mask));
+}
+#endif
+
+static void __ref create_mapping(phys_addr_t phys, unsigned long virt,
+				  phys_addr_t size,
+				  pgprot_t sect_prot, pgprot_t pte_prot)
 {
 	if (virt < VMALLOC_START) {
 		pr_warn("BUG: not creating mapping for %pa@0x%016lx - outside kernel range\n",
 			&phys, virt);
 		return;
 	}
-	__create_mapping(pgd_offset_k(virt & PAGE_MASK), phys, virt, size, 0);
+
+	return __create_mapping(pgd_offset_k(virt & PAGE_MASK), phys, virt,
+				size, sect_prot, pte_prot, 0, true);
 }
 
-void __init create_id_mapping(phys_addr_t addr, phys_addr_t size, int map_io)
+static void __ref create_mapping_late(phys_addr_t phys, unsigned long virt,
+				  phys_addr_t size,
+				  pgprot_t sect_prot, pgprot_t pte_prot)
 {
-	if ((addr >> PGDIR_SHIFT) >= ARRAY_SIZE(idmap_pg_dir)) {
-		pr_warn("BUG: not creating id mapping for %pa\n", &addr);
+	if (virt < VMALLOC_START) {
+		pr_warn("BUG: not creating mapping for %pa@0x%016lx - outside kernel range\n",
+			&phys, virt);
 		return;
 	}
-	__create_mapping(&idmap_pg_dir[pgd_index(addr)],
-			 addr, addr, size, map_io);
+
+	return __create_mapping(pgd_offset_k(virt & PAGE_MASK), phys, virt,
+				size, sect_prot, pte_prot, 0, false);
+}
+
+#ifdef CONFIG_DEBUG_RODATA
+static void __init __map_memblock(phys_addr_t start, phys_addr_t end)
+{
+	/*
+	 * Set up the executable regions using the existing section mappings
+	 * for now. This will get more fine grained later once all memory
+	 * is mapped
+	 */
+	unsigned long kernel_x_start = round_down(__pa(_stext), SECTION_SIZE);
+	unsigned long kernel_x_end = round_up(__pa(__init_end), SECTION_SIZE);
+
+	if (end < kernel_x_start) {
+		create_mapping(start, __phys_to_virt(start),
+			end - start, PROT_SECT_NORMAL, PAGE_KERNEL);
+	} else if (start >= kernel_x_end) {
+		create_mapping(start, __phys_to_virt(start),
+			end - start, PROT_SECT_NORMAL, PAGE_KERNEL);
+	} else {
+		if (start < kernel_x_start)
+			create_mapping(start, __phys_to_virt(start),
+				kernel_x_start - start,
+				PROT_SECT_NORMAL,
+				PAGE_KERNEL);
+		create_mapping(kernel_x_start,
+				__phys_to_virt(kernel_x_start),
+				kernel_x_end - kernel_x_start,
+				PROT_SECT_NORMAL_EXEC, PAGE_KERNEL_EXEC);
+		if (kernel_x_end < end)
+			create_mapping(kernel_x_end,
+				__phys_to_virt(kernel_x_end),
+				end - kernel_x_end,
+				PROT_SECT_NORMAL,
+				PAGE_KERNEL);
+	}
+
+}
+#else
+static void __init __map_memblock(phys_addr_t start, phys_addr_t end)
+{
+	create_mapping(start, __phys_to_virt(start), end - start,
+			PROT_SECT_NORMAL_EXEC, PAGE_KERNEL_EXEC);
 }
+#endif
 
 static void __init map_mem(void)
 {
@@ -328,14 +503,69 @@ static void __init map_mem(void)
 			memblock_set_current_limit(limit);
 		}
 #endif
-
-		create_mapping(start, __phys_to_virt(start), end - start);
+		__map_memblock(start, end);
 	}
 
 	/* Limit no longer required. */
 	memblock_set_current_limit(MEMBLOCK_ALLOC_ANYWHERE);
 }
 
+void __init fixup_executable(void)
+{
+#ifdef CONFIG_DEBUG_RODATA
+	/* now that we are actually fully mapped, make the start/end more fine grained */
+	if (!IS_ALIGNED((unsigned long)_stext, SECTION_SIZE)) {
+		unsigned long aligned_start = round_down(__pa(_stext),
+							SECTION_SIZE);
+
+		create_mapping(aligned_start, __phys_to_virt(aligned_start),
+				__pa(_stext) - aligned_start,
+				PROT_SECT_NORMAL,
+				PAGE_KERNEL);
+	}
+
+	if (!IS_ALIGNED((unsigned long)__init_end, SECTION_SIZE)) {
+		unsigned long aligned_end = round_up(__pa(__init_end),
+							SECTION_SIZE);
+		create_mapping(__pa(__init_end), (unsigned long)__init_end,
+				aligned_end - __pa(__init_end),
+				PROT_SECT_NORMAL,
+				PAGE_KERNEL);
+	}
+#endif
+}
+
+#ifdef CONFIG_DEBUG_RODATA
+void mark_rodata_ro(void)
+{
+	create_mapping_late(__pa(_stext), (unsigned long)_stext,
+				(unsigned long)_etext - (unsigned long)_stext,
+				PROT_SECT_NORMAL_EXEC | PMD_SECT_RDONLY,
+				PAGE_KERNEL_EXEC | PTE_RDONLY);
+
+}
+#endif
+
+static int __flush_mappings(void *unused)
+{
+	flush_tlb_kernel_range((unsigned long)__init_begin,
+				(unsigned long)__init_end);
+	return 0;
+}
+
+void __ref fixup_init(void)
+{
+	phys_addr_t start = __pa(__init_begin);
+	phys_addr_t end = __pa(__init_end);
+
+	create_mapping_late(start, (unsigned long)__init_begin,
+			end - start,
+			PROT_SECT_NORMAL,
+			PAGE_KERNEL);
+	if (!IS_ALIGNED(start, SECTION_SIZE) || !IS_ALIGNED(end, SECTION_SIZE))
+		stop_machine(__flush_mappings, NULL, NULL);
+}
+
 /*
  * paging_init() sets up the page tables, initialises the zone memory
  * maps and sets up the zero page.
@@ -345,6 +575,7 @@ void __init paging_init(void)
 	void *zero_page;
 
 	map_mem();
+	fixup_executable();
 
 	/*
 	 * Finally flush the caches and tlb to ensure that we're in a
-- 
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