[PATCH v2] arm64: mm: fix location of _etext

[Ard Biesheuvel]

As Kees Cook notes in the ARM counterpart of this patch [0]:

  The _etext position is defined to be the end of the kernel text code,
  and should not include any part of the data segments. This interferes
  with things that might check memory ranges and expect executable code
  up to _etext.

In particular, Kees is referring to the HARDENED_USERCOPY patch set [1],
which rejects attempts to call copy_to_user() on kernel ranges containing
executable code, but does allow access to the .rodata segment. Regardless
of whether one may or may not agree with the distinction, it makes sense
for _etext to have the same meaning across architectures.

So let's put _etext where it belongs, between .text and .rodata, and fix
up existing references to use __init_begin instead, which unlike _end_rodata
includes the exception and notes sections as well.

The _etext references in kaslr.c are left untouched, since its references
to [_stext, _etext) are meant to capture potential jump instruction targets,
and so disregarding .rodata is actually an improvement here.

[0] http://article.gmane.org/gmane.linux.kernel/2245084
[1] http://thread.gmane.org/gmane.linux.kernel.hardened.devel/2502

Reported-by: Kees Cook <keescook at chromium.org>
Reviewed-by: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
v2: - preserve reference to _etext in init_mm, which redefines the init_mm
      code region to no longer cover .rodata as well
    - update comments regarding overlap of text to refer to 'text/rodata'
      instead
    - added Mark's R-b

 arch/arm64/kernel/setup.c       |  2 +-
 arch/arm64/kernel/vmlinux.lds.S |  7 ++++---
 arch/arm64/mm/init.c            |  4 ++--
 arch/arm64/mm/mmu.c             | 20 ++++++++++----------
 4 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 3279defabaa2..c1509e654190 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -202,7 +202,7 @@ static void __init request_standard_resources(void)
 	struct resource *res;
 
 	kernel_code.start   = virt_to_phys(_text);
-	kernel_code.end     = virt_to_phys(_etext - 1);
+	kernel_code.end     = virt_to_phys(__init_begin - 1);
 	kernel_data.start   = virt_to_phys(_sdata);
 	kernel_data.end     = virt_to_phys(_end - 1);
 
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 435e820e898d..0de7be4f1a9d 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -131,12 +131,13 @@ SECTIONS
 	}
 
 	. = ALIGN(SEGMENT_ALIGN);
-	RO_DATA(PAGE_SIZE)		/* everything from this point to */
-	EXCEPTION_TABLE(8)		/* _etext will be marked RO NX   */
+	_etext = .;			/* End of text section */
+
+	RO_DATA(PAGE_SIZE)		/* everything from this point to     */
+	EXCEPTION_TABLE(8)		/* __init_begin will be marked RO NX */
 	NOTES
 
 	. = ALIGN(SEGMENT_ALIGN);
-	_etext = .;			/* End of text and rodata section */
 	__init_begin = .;
 
 	INIT_TEXT_SECTION(8)
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index d45f8627012c..7bc5bed0220a 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -430,9 +430,9 @@ void __init mem_init(void)
 	pr_cont("    vmalloc : 0x%16lx - 0x%16lx   (%6ld GB)\n",
 		MLG(VMALLOC_START, VMALLOC_END));
 	pr_cont("      .text : 0x%p" " - 0x%p" "   (%6ld KB)\n",
-		MLK_ROUNDUP(_text, __start_rodata));
+		MLK_ROUNDUP(_text, _etext));
 	pr_cont("    .rodata : 0x%p" " - 0x%p" "   (%6ld KB)\n",
-		MLK_ROUNDUP(__start_rodata, _etext));
+		MLK_ROUNDUP(__start_rodata, __init_begin));
 	pr_cont("      .init : 0x%p" " - 0x%p" "   (%6ld KB)\n",
 		MLK_ROUNDUP(__init_begin, __init_end));
 	pr_cont("      .data : 0x%p" " - 0x%p" "   (%6ld KB)\n",
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 0f85a46c3e18..c356f7b84d4d 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -386,14 +386,14 @@ static void create_mapping_late(phys_addr_t phys, unsigned long virt,
 static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end)
 {
 	unsigned long kernel_start = __pa(_text);
-	unsigned long kernel_end = __pa(_etext);
+	unsigned long kernel_end = __pa(__init_begin);
 
 	/*
 	 * Take care not to create a writable alias for the
 	 * read-only text and rodata sections of the kernel image.
 	 */
 
-	/* No overlap with the kernel text */
+	/* No overlap with the kernel text/rodata */
 	if (end < kernel_start || start >= kernel_end) {
 		__create_pgd_mapping(pgd, start, __phys_to_virt(start),
 				     end - start, PAGE_KERNEL,
@@ -402,7 +402,7 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
 	}
 
 	/*
-	 * This block overlaps the kernel text mapping.
+	 * This block overlaps the kernel text/rodata mappings.
 	 * Map the portion(s) which don't overlap.
 	 */
 	if (start < kernel_start)
@@ -417,7 +417,7 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
 				     early_pgtable_alloc);
 
 	/*
-	 * Map the linear alias of the [_text, _etext) interval as
+	 * Map the linear alias of the [_text, __init_begin) interval as
 	 * read-only/non-executable. This makes the contents of the
 	 * region accessible to subsystems such as hibernate, but
 	 * protects it from inadvertent modification or execution.
@@ -449,14 +449,14 @@ void mark_rodata_ro(void)
 {
 	unsigned long section_size;
 
-	section_size = (unsigned long)__start_rodata - (unsigned long)_text;
+	section_size = (unsigned long)_etext - (unsigned long)_text;
 	create_mapping_late(__pa(_text), (unsigned long)_text,
 			    section_size, PAGE_KERNEL_ROX);
 	/*
-	 * mark .rodata as read only. Use _etext rather than __end_rodata to
-	 * cover NOTES and EXCEPTION_TABLE.
+	 * mark .rodata as read only. Use __init_begin rather than __end_rodata
+	 * to cover NOTES and EXCEPTION_TABLE.
 	 */
-	section_size = (unsigned long)_etext - (unsigned long)__start_rodata;
+	section_size = (unsigned long)__init_begin - (unsigned long)__start_rodata;
 	create_mapping_late(__pa(__start_rodata), (unsigned long)__start_rodata,
 			    section_size, PAGE_KERNEL_RO);
 }
@@ -499,8 +499,8 @@ static void __init map_kernel(pgd_t *pgd)
 {
 	static struct vm_struct vmlinux_text, vmlinux_rodata, vmlinux_init, vmlinux_data;
 
-	map_kernel_segment(pgd, _text, __start_rodata, PAGE_KERNEL_EXEC, &vmlinux_text);
-	map_kernel_segment(pgd, __start_rodata, _etext, PAGE_KERNEL, &vmlinux_rodata);
+	map_kernel_segment(pgd, _text, _etext, PAGE_KERNEL_EXEC, &vmlinux_text);
+	map_kernel_segment(pgd, __start_rodata, __init_begin, PAGE_KERNEL, &vmlinux_rodata);
 	map_kernel_segment(pgd, __init_begin, __init_end, PAGE_KERNEL_EXEC,
 			   &vmlinux_init);
 	map_kernel_segment(pgd, _data, _end, PAGE_KERNEL, &vmlinux_data);
-- 
2.7.4