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Date: Tue, 14 Feb 2017 15:57:05 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC: <linux-arm-kernel@...ts.infradead.org>, <catalin.marinas@....com>,
<will.deacon@....com>, <labbott@...oraproject.org>,
<kvmarm@...ts.cs.columbia.edu>, <marc.zyngier@....com>,
<andre.przywara@....com>, <Suzuki.Poulose@....com>, <james.morse@....com>,
<keescook@...omium.org>, <kernel-hardening@...ts.openwall.com>, <nd@....com>
Subject: Re: [PATCH v2 4/5] arm64: mmu: map .text as read-only from the outset
On Sat, Feb 11, 2017 at 08:23:05PM +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.
>
> Reviewed-by: Laura Abbott <labbott@...hat.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
This generally looks good.
One effect of this is that even with rodata=off, external debuggers
can't install SW breakpoints via the executable mapping.
We might want to allow that to be overridden. e.g. make rodata= an
early param, and switch the permissions based on that in map_kernel(),
e.g. have:
pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX
: PAGE_KERNEL_EXEC);
... and use that for .text and .init.text by default.
Thanks,
Mark.
> ---
> arch/arm64/mm/mmu.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 7ed981c7f4c0..e97f1ce967ec 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -442,9 +442,6 @@ void mark_rodata_ro(void)
> {
> unsigned long section_size;
>
> - section_size = (unsigned long)_etext - (unsigned long)_text;
> - create_mapping_late(__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.
> @@ -484,7 +481,7 @@ 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);
> + map_kernel_segment(pgd, _text, _etext, PAGE_KERNEL_ROX, &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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