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Message-Id: <651D9CBB-3E64-41CE-BF85-D2FF0CB927B7@linaro.org>
Date: Tue, 14 Feb 2017 16:15:11 +0000
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Mark Rutland <mark.rutland@....com>
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 14 Feb 2017, at 15:57, Mark Rutland <mark.rutland@....com> wrote:
>
>> 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.
>
Interesting. For the sake of my education, could you elaborate on how that works under the hood?
> 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.
>
>
Is there any way we could restrict this privilege to external debuggers? Having trivial 'off' switches for security features makes me feel uneasy (although this is orthogonal to this patch)
>> ---
>> 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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