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

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Message-ID: <CAKv+Gu8r0Fq0ZBtM5BCOK8=Bmn6OxF5OhNHFRz_Wh2eac=1hHQ@mail.gmail.com>
Date:	Thu, 18 Feb 2016 12:55:50 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	PaX Team <pageexec@...email.hu>
Cc:	Laura Abbott <labbott@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Rutland <mark.rutland@....com>,
	Jeremy Linton <jeremy.linton@....com>,
	Arnd Bergmann <arnd@...db.de>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lkdtm: add test for executing .rodata

On 18 February 2016 at 12:34, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> On 18 February 2016 at 11:32, PaX Team <pageexec@...email.hu> wrote:
>> On 17 Feb 2016 at 12:29, Kees Cook wrote:
>>
>>> >> +static void __attribute__((__section__(".rodata,\"a\",@progbits#")))
>>> >> +do_nothing_rodata(void)
>>> >> +{
>>> >> +       return;
>>> >> +}
>>> >> +
>>> >
>>> >
>>> >>
>>> >
>>> > This doesn't cross compile for me on arm64 with two different toolchains
>>> >
>>> > CC drivers/misc/lkdtm.o
>>> > /tmp/ccHzIWIx.s: Assembler messages:
>>> > /tmp/ccHzIWIx.s:21: Error: junk at end of line, first unrecognized character
>>> > is `#'
>>> > /tmp/ccHzIWIx.s: Error: unaligned opcodes detected in executable segment
>>> > scripts/Makefile.build:258: recipe for target 'drivers/misc/lkdtm.o' failed
>>> > make[2]: *** [drivers/misc/lkdtm.o] Error 1
>>> > scripts/Makefile.build:407: recipe for target 'drivers/misc' failed
>>> > make[1]: *** [drivers/misc] Error 2
>>> > Makefile:950: recipe for target 'drivers' failed
>>> > make: *** [drivers] Error 2
>>> >
>>> > I don't know the assembler well enough to give any insight.
>>>
>>> Hm, bummer. I was trying to get fancy with the function forced into
>>> .rodata by trying to force the bits. Looks like "#" is not seen as a
>>> comment character by the toolchain you're using.
>>>
>>> Anyone else successfully done tricks like this?
>>
>> wouldn't it be a better and more generic/reusable approach to
>>
>> #define __ro_text __attribute__((__section__(".rodata.text")))
>>
>> and move this function there by the linker script similar to how it's done
>> for other code that goes into special sections?
>>
>
> We have __section() as an alias for __attribute__((__section__())), so
> we could use that instead.
>
> 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
>
> I wonder if we could get away with doing something like
>
> AFLAGS_lkdtm.o += -Wa,-W
>
> here? This just hides the warnings, but may result in the .rodata
> section in the vmlinux file to have X permissions as well. I don't
> think anyone uses an ELF loader to load their kernel, but who knows
> ...
>
> That only matters when lkdtm is a module, btw

... is NOT a module, obviously