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Re: [kernel-hardening] Re: [PATCH] lkdtm: add test for executing .rodata

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  Re: [kernel-hardening] Re: [PATCH] lkdtm: add test for executing  .rodata David Brown <david.brown@linaro.org> - 2016-02-23 21:40 +0100
    Re: [kernel-hardening] Re: [PATCH] lkdtm: add test for executing .rodata Kees Cook <keescook@chromium.org> - 2016-02-23 22:00 +0100

#1341008 — Re: [kernel-hardening] Re: [PATCH] lkdtm: add test for executing .rodata

On Mon, Feb 22, 2016 at 12:46:28PM -0800, Kees Cook wrote:
>On Thu, Feb 18, 2016 at 1:27 PM, PaX Team <pageexec@freemail.hu> wrote:
>> On 18 Feb 2016 at 12:34, Ard Biesheuvel wrote:
>>
>>> However, that does not fix the issue Kees is trying to solve, where a
>>> .rodata section is emitted with the "x" bit set, which causes the
>>> linker to complain:
>>>
>>> /tmp/cc50ffWw.s: Assembler messages:
>>> /tmp/cc50ffWw.s:2: Warning: setting incorrect section attributes for
>>> .rodata.text
>>
>> in that case why not use a top-level asm statement to set the section
>> and its attributes (and compile the file with fno-toplevel-reorder)?
>
>GCC really wants to declare the section. :(

Why not then just use its mechanism to set the section on the code?

static void do_nothing_rodata(void)
	__attribute__((section(".rodata")))
{
	return;
}

David

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#1341024 — Re: [kernel-hardening] Re: [PATCH] lkdtm: add test for executing .rodata

On Tue, Feb 23, 2016 at 12:31 PM, David Brown <david.brown@linaro.org> wrote:
> On Mon, Feb 22, 2016 at 12:46:28PM -0800, Kees Cook wrote:
>>
>> On Thu, Feb 18, 2016 at 1:27 PM, PaX Team <pageexec@freemail.hu> wrote:
>>>
>>> On 18 Feb 2016 at 12:34, Ard Biesheuvel wrote:
>>>
>>>> However, that does not fix the issue Kees is trying to solve, where a
>>>> .rodata section is emitted with the "x" bit set, which causes the
>>>> linker to complain:
>>>>
>>>> /tmp/cc50ffWw.s: Assembler messages:
>>>> /tmp/cc50ffWw.s:2: Warning: setting incorrect section attributes for
>>>> .rodata.text
>>>
>>>
>>> in that case why not use a top-level asm statement to set the section
>>> and its attributes (and compile the file with fno-toplevel-reorder)?
>>
>>
>> GCC really wants to declare the section. :(
>
>
> Why not then just use its mechanism to set the section on the code?

It's noted in the quoted section above. :) The problem is that gcc emits:

.section ".rodata","ax",@progbits

But we need the flags to be "a" not "ax". (And I'd note that even "@"
is arch-specific. % is used when @ is a comment start character.)

objcopy can get me the "a", but it seems to require renaming lkdtm.c,
which I really don't want to do.

-Kees

>
> static void do_nothing_rodata(void)
>         __attribute__((section(".rodata")))
> {
>         return;
> }
>
> David



-- 
Kees Cook
Chrome OS & Brillo Security

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