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Message-ID: <CAGXu5jLpj=ue49_N+LOYvtZcU3rpFR5=WTG15ySDOWprvRzFUA@mail.gmail.com>
Date: Tue, 16 Feb 2016 10:10:01 -0800
From: Kees Cook <keescook@...omium.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Jeremy Linton <jeremy.linton@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>, Laura Abbott <laura@...bott.name>,
"Suzuki K. Poulose" <suzuki.poulose@....com>, Will Deacon <will.deacon@....com>,
Catalin Marinas <catalin.marinas@....com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH] arm64: mm: Mark .rodata as RO
On Fri, Feb 12, 2016 at 10:25 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Fri, Feb 12, 2016 at 10:13:19AM -0600, Jeremy Linton wrote:
>> Currently the .rodata section is actually still executable when DEBUG_RODATA
>> is enabled. This changes that so the .rodata is actually read only, no execute.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
Yikes, good catch. Is anyone running the lkdtm tests that check these things?
Reviewed-by: Kees Cook <keescook@...omium.org>
>> ---
>> arch/arm64/kernel/vmlinux.lds.S | 5 +++--
>> arch/arm64/mm/mmu.c | 14 +++++++++++---
>> 2 files changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
>> index ab4e436..2e2c053 100644
>> --- a/arch/arm64/kernel/vmlinux.lds.S
>> +++ b/arch/arm64/kernel/vmlinux.lds.S
>> @@ -114,8 +114,9 @@ SECTIONS
>> *(.got) /* Global offset table */
>> }
>>
>> - RO_DATA(PAGE_SIZE)
>> - EXCEPTION_TABLE(8)
>> + ALIGN_DEBUG_RO_MIN(0)
>> + RO_DATA(PAGE_SIZE) /* everything from this point to */
>> + EXCEPTION_TABLE(8) /* _etext will be marked RO NX */
>> NOTES
>>
>> ALIGN_DEBUG_RO_MIN(PAGE_SIZE)
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index ab69a99..a3f4112 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -453,10 +453,18 @@ static void __init map_mem(pgd_t *pgd)
>> #ifdef CONFIG_DEBUG_RODATA
>> void mark_rodata_ro(void)
>> {
>> - create_mapping_late(__pa(_stext), (unsigned long)_stext,
>> - (unsigned long)_etext - (unsigned long)_stext,
>> - PAGE_KERNEL_ROX);
>> + unsigned long section_size;
>>
>> + section_size = (unsigned long)__start_rodata - (unsigned long)_stext;
>> + create_mapping_late(__pa(_stext), (unsigned long)_stext,
>> + section_size, PAGE_KERNEL_ROX);
>> + /*
>> + * mark .rodata as read only. Use _etext rather than __end_rodata to
>> + * cover NOTES and EXCEPTION_TABLE.
>> + */
>> + section_size = (unsigned long)_etext - (unsigned long)__start_rodata;
>> + create_mapping_late(__pa(__start_rodata), (unsigned long)__start_rodata,
>> + section_size, PAGE_KERNEL_RO);
>> }
>
> As you pointed out in the other thread, we'll also need to update
> map_kernel to use equivalent chunks for .text and .rodata.
>
> I think we can probably make .rodata RO from the outset in map_kernel,
> too.
There's a benefit to marking it late in that .rodata can live in the
same (upcoming) section as .data..ro_after_init, which is marked ro
after mark_rodata_ro() to help constify more things.
> Could you please update mem_init to log .text and .rodata separately?
>
> It looks like some core code makes assumptions about _etext, so I guess
> that has to cover .rodata regardless.
>
> Otherwise, looks good!
>
> Thanks,
> Mark.
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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