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

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Message-ID: <CAKv+Gu_ABQa4L7ZzHMD+-BPLtJf_UO9TfLy0jKFLfCsyQE6dLQ@mail.gmail.com>
Date:	Thu, 18 Feb 2016 12:34:50 +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 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