[kernel-hardening] Re: [PATCH v4 2/2] extable: verify address is read-only

Eddie Kovsky
2017-03-31T03:09:34Z

On 03/30/17, Kees Cook wrote:
> On Wed, Mar 29, 2017 at 8:27 PM, Jessica Yu <jeyu@redhat.com> wrote:
> > +++ Eddie Kovsky [28/03/17 21:28 -0600]:
> >
> >> On 03/27/17, Kees Cook wrote:
> >>>
> >>> On Mon, Mar 27, 2017 at 1:43 AM, kbuild test robot <lkp@intel.com> wrote:
> >>> > Hi Eddie,
> >>> >
> >>> > [auto build test ERROR on next-20170323]
> >>> > [cannot apply to linus/master linux/master jeyu/modules-next v4.9-rc8
> >>> > v4.9-rc7 v4.9-rc6 v4.11-rc4]
> >>> > [if your patch is applied to the wrong git tree, please drop us a note
> >>> > to help improve the system]
> >>> >
> >>> > url:
> >>> > https://github.com/0day-ci/linux/commits/Eddie-Kovsky/module-verify-address-is-read-only/20170327-142922
> >>> > config: blackfin-BF561-EZKIT-SMP_defconfig (attached as .config)
> >>> > compiler: bfin-uclinux-gcc (GCC) 6.2.0
> >>> > reproduce:
> >>> > wget
> >>> > https://raw.githubusercontent.com/01org/lkp-tests/master/sbin/make.cross -O
> >>> > ~/bin/make.cross
> >>> > chmod +x ~/bin/make.cross
> >>> > # save the attached .config to linux build tree
> >>> > make.cross ARCH=blackfin
> >>> >
> >>> > All errors (new ones prefixed by >>):
> >>> >
> >>> > kernel/built-in.o: In function `core_kernel_rodata':
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__start_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__start_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__end_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__end_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__start_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__start_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__end_data_ro_after_init'
> >>> >>> kernel/extable.c:169: undefined reference to
> >>> >>> `__end_data_ro_after_init'
> >>>
> >>> Hm, I'm confused about this. blackfin includes
> >>> include/asm-generic-vmlinux.lds.h and uses the RO_DATA macro (which
> >>> resolves to RO_DATA_SECTION to RO_AFTER_INIT_DATA which defines
> >>> __[start|end]_data_ro_after_init.
> >>>
> >>> Also, it seems that commit d7c19b066dcf4bd19c4385e8065558d4e74f9e73
> >>> ("mm: kmemleak: scan .data.ro_after_init") added a potentially
> >>> redundant section name (s390 already calls this
> >>> __[start|end]_ro_after_init). I'd like to get this cleaned up, since
> >>> having multiple names for the same thing is confusing:
> >>>
> >>> diff --git a/arch/s390/kernel/vmlinux.lds.S
> >>> b/arch/s390/kernel/vmlinux.lds.S
> >>> index 000e6e91f6a0..3667d20e997f 100644
> >>> --- a/arch/s390/kernel/vmlinux.lds.S
> >>> +++ b/arch/s390/kernel/vmlinux.lds.S
> >>> @@ -62,9 +62,11 @@ SECTIONS
> >>>
> >>> . = ALIGN(PAGE_SIZE);
> >>> __start_ro_after_init = .;
> >>> + __start_data_ro_after_init = .;
> >>> .data..ro_after_init : {
> >>> *(.data..ro_after_init)
> >>> }
> >>> + __end_data_ro_after_init = .;
> >>> EXCEPTION_TABLE(16)
> >>> . = ALIGN(PAGE_SIZE);
> >>> __end_ro_after_init = .;
> >>>
> >>> And it seems that this hunk is wrong (__end_ro_after_init includes
> >>> s390's exception table, etc). I think we should remove the
> >>> ..._data_... name and use s390's name.
> >>>
> >>> I'll send an adjustment patch, but we'll still need to deal with
> >>> blackfin.
> >>>
> >>> -Kees
> >>>
> >>
> >> Kees
> >>
> >> I applied your patch (mm: fix section name for .data..ro_after_init) and
> >> changed the new function in extable.c to use __[start|end]_ro_after_init
> >> instead. The new version still builds without errors on x86, which isn't
> >> surprising.
> >>
> >> I've cross compiled this for blackfin and I'm able to reproduce the
> >> build error. I'm still not sure why. As you pointed out, blackfin does
> >> appear to use 'include/asm-generic/vmlinux.lds.h'.
> >
> >
> > This appears to be because blackfin is one of the 2 arches that
> > prepends an underscore '_' to all symbols defined in C. I noticed that
> > __{start,end}_data_ro_after_init in vmlinux.lds.h are not wrapped with
> > VMLINUX_SYMBOL() which adds the necessary prefix for arches that have
> > HAVE_UNDERSCORE_SYMBOL_PREFIX, hence the undefined reference.
>
> Argh. Thank you for catching this! Yeah, that would have taken me
> forever to find.
>
> > The below patch fixed the build error for me, if it works for you then
> > I can send a formal patch.
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h
> > b/include/asm-generic/vmlinux.lds.h
> > index 4e09b28..7b262f7 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -260,9 +260,9 @@
> > */
> > #ifndef RO_AFTER_INIT_DATA
> > #define RO_AFTER_INIT_DATA \
> > - __start_data_ro_after_init = .; \
> > + VMLINUX_SYMBOL(__start_data_ro_after_init) = .; \
> > *(.data..ro_after_init) \
> > - __end_data_ro_after_init = .;
> > + VMLINUX_SYMBOL(__end_data_ro_after_init) = .;
> > #endif
>
> I don't have a blackfin cross-compiler set up, but I'm sure that'll
> fix it. If you can, please base it on -next, since I rename
> __[start|end]_data_ro_after_init to __[start|end]_ro_after_init (to
> match the existing s390 symbols of the same purpose):
>
> https://lkml.org/lkml/2017/3/27/685
>
I applied Jessica's patch and tested this again with a blackfin cross-compiler.
It fixes the error building extable.c.

> akpm is carrying that patch, so this follow-up should likely go to him too.
>
> Thanks!
>
> -Kees
>
> --
> Kees Cook
> Pixel Security