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

[Laura Abbott]

On 06/08/2016 02:26 PM, Kees Cook wrote:
> On Wed, Jun 8, 2016 at 8:46 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > On Tue, Jun 7, 2016 at 6:02 PM, Laura Abbott <labbott@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > ---
> > > >  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