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Date: Tue, 7 Mar 2017 14:10:14 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: linux-arm-kernel@...ts.infradead.org,
kernel-hardening@...ts.openwall.com, catalin.marinas@....com,
will.deacon@....com, labbott@...oraproject.org,
kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
keescook@...omium.org, andre.przywara@....com, james.morse@....com,
suzuki.poulose@....com
Subject: Re: [PATCH v4 4/6] arm64: mmu: map .text as read-only from the outset
On Sat, Mar 04, 2017 at 02:30:46PM +0000, Ard Biesheuvel wrote:
> 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@...hat.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
> arch/arm64/mm/mmu.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index df377fbe464e..edd982f88714 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 +461,9 @@ 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);
> + pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;
> +
It might be worth having a comment as to why, e.g.
/*
* External debuggers may need to write directly to the text
* mapping to install SW breakpoints. Allow this (only) when
* explicitly requested with rodata=off.
*/
... otherwise this looks fine. FWIW, either way:
Reviewed-by: Mark Rutland <mark.rutland@....com>
Thanks,
Mark.
> + 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
>
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