linux-kernel - Re: [PATCH 3/5] lkdtm: add function for testing .rodata section

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Message-ID: <0fdbafc6-21ce-636c-038c-1e870ebf75d6@redhat.com>
Date:	Thu, 9 Jun 2016 18:14:36 -0700
From:	Laura Abbott <labbott@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Guenter Roeck <linux@...ck-us.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] lkdtm: add function for testing .rodata section

On 06/08/2016 02:26 PM, Kees Cook wrote:
> On Wed, Jun 8, 2016 at 8:46 AM, Kees Cook <keescook@...omium.org> wrote:
>> On Tue, Jun 7, 2016 at 6:02 PM, Laura Abbott <labbott@...hat.com> wrote:
>>> On 06/07/2016 02:57 PM, Kees Cook wrote:
>>>>
>>>> This adds a function that lives in the .rodata section. The section
>>>> flags are corrected using objcopy since there is no way with gcc to
>>>> declare section flags in an architecture-agnostic way.
>>>>
>>>
>>> Permit me to be the bearer of bad architecture news once again. With
>>> arm64 cross compiler (both Fedora 6.1.1 and Linaro 5.1)
>>>
>>>   CC      drivers/misc/lkdtm_rodata.o
>>>   OBJCOPY drivers/misc/lkdtm_rodata_objcopy.o
>>>   LD      drivers/misc/lkdtm.o
>>> drivers/misc/lkdtm_rodata_objcopy.o: file not recognized: File format not
>>> recognized
>>> scripts/Makefile.build:423: recipe for target 'drivers/misc/lkdtm.o' failed
>>> make[2]: *** [drivers/misc/lkdtm.o] Error 1
>>> scripts/Makefile.build:440: recipe for target 'drivers/misc' failed
>>> make[1]: *** [drivers/misc] Error 2
>>> Makefile:985: recipe for target 'drivers' failed
>>> make: *** [drivers] Error 2
>>>
>>>
>>> As far as I can tell this is because arm64 defines OBJCOPYFLAGS and they get
>>> propagated to objcopy
>>>
>>> aarch64-linux-gnu-objcopy -O binary -R .note -R .note.gnu.build-id -R
>>> .comment
>>>         -S --set-section-flags .text=alloc,readonly
>>>         --rename-section .text=.rodata drivers/misc/lkdtm_rodata.o
>>> drivers/misc/lkdtm_rodata_objcopy.o
>>>
>>> vs x86
>>>
>>> objcopy  --set-section-flags .text=alloc,readonly --rename-section
>>> .text=.rodata
>>>         drivers/misc/lkdtm_rodata.o drivers/misc/lkdtm_rodata_objcopy.o
>>>
>>>
>>> specifically it's the -O binary that seems to break things, the same failure
>>> happens on x86 as well with the the same commands. It works if I clear out
>>> the OBJCOPYFLAGS variable first but I don't think that's the correct way to
>>> fix this.
>>>
>>> Thanks,
>>> Laura
>>>
>>>
>>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>>> ---
>>>>  drivers/misc/Makefile       |  7 +++++++
>>>>  drivers/misc/lkdtm.h        |  6 ++++++
>>>>  drivers/misc/lkdtm_core.c   | 24 +++++++++++++++++-------
>>>>  drivers/misc/lkdtm_rodata.c | 10 ++++++++++
>>>>  4 files changed, 40 insertions(+), 7 deletions(-)
>>>>  create mode 100644 drivers/misc/lkdtm.h
>>>>  create mode 100644 drivers/misc/lkdtm_rodata.c
>>>>
>>>> diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
>>>> index c3cb6ad8cc37..b2d3d68dfa22 100644
>>>> --- a/drivers/misc/Makefile
>>>> +++ b/drivers/misc/Makefile
>>>> @@ -59,3 +59,10 @@ obj-$(CONFIG_CXL_BASE)               += cxl/
>>>>  obj-$(CONFIG_PANEL)             += panel.o
>>>>
>>>>  lkdtm-$(CONFIG_LKDTM)          += lkdtm_core.o
>>>> +lkdtm-$(CONFIG_LKDTM)          += lkdtm_rodata_objcopy.o
>>>> +
>
> If I add:
>
> OBJCOPYFLAGS :=
>
> here, it seems to fix things...
>

I can confirm that this works on arm64

# echo EXEC_RODATA > /sys/kernel/debug/provoke-crash/DIRECT
[   21.725514] lkdtm: Performing direct entry EXEC_RODATA
[   21.725890] lkdtm: attempting ok execution at ffff0000084c0e08
[   21.726030] lkdtm: attempting bad execution at ffff000008880700
[   21.726401] Bad mode in Synchronous Abort handler detected on CPU2, code 0x8400000e -- IABT (current EL)
[   21.726847] CPU: 2 PID: 998 Comm: sh Not tainted 4.7.0-rc2+ #13

I wish the exception was clearer what the actual error was. I might propose a
patch to make it more obvious.

Thanks,
Laura