[PATCH v6 04/11] arm64: mmu: map .text as read-only from the outset

Ard Biesheuvel
2017-03-09T20:52:02Z

Now that alternatives patching code no longer relies on the primary
mapping of .text being writable, we can remove the code that removes
the writable permissions post-init time, and map it read-only from
the outset.

To preserve the existing behavior under rodata=off, which is relied
upon by external debuggers to manage software breakpoints (as pointed
out by Mark), add an early_param() check for rodata=, and use RWX
permissions if it set to 'off'.

Reviewed-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/arm64/mm/mmu.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index df377fbe464e..300e98e8cd63 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -416,9 +416,6 @@ void mark_rodata_ro(void)
{
unsigned long section_size;

- section_size = (unsigned long)_etext - (unsigned long)_text;
- update_mapping_prot(__pa_symbol(_text), (unsigned long)_text,
- section_size, PAGE_KERNEL_ROX);
/*
* mark .rodata as read only. Use __init_begin rather than __end_rodata
* to cover NOTES and EXCEPTION_TABLE.
@@ -451,6 +448,12 @@ static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end,
vm_area_add_early(vma);
}

+static int __init parse_rodata(char *arg)
+{
+ return strtobool(arg, &rodata_enabled);
+}
+early_param("rodata", parse_rodata);
+
/*
* Create fine-grained mappings for the kernel.
*/
@@ -458,7 +461,14 @@ 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, _etext, PAGE_KERNEL_EXEC, &vmlinux_text);
+ /*
+ * External debuggers may need to write directly to the text
+ * mapping to install SW breakpoints. Allow this (only) when
+ * explicitly requested with rodata=off.
+ */
+ pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;
+
+ map_kernel_segment(pgd, _text, _etext, text_prot, &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);
--
2.7.4