linux-kernel - Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

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Message-ID: <201910220929.ADF807CC@keescook>
Date:   Tue, 22 Oct 2019 09:30:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Laura Abbott <labbott@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

On Tue, Oct 22, 2019 at 05:28:27PM +0100, Mark Rutland wrote:
> On Fri, Oct 18, 2019 at 09:10:21AM -0700, Sami Tolvanen wrote:
> > This change adds generic support for Clang's Shadow Call Stack, which
> > uses a shadow stack to protect return addresses from being overwritten
> > by an attacker. Details are available here:
> > 
> >   https://clang.llvm.org/docs/ShadowCallStack.html
> > 
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > ---
> >  Makefile                       |   6 ++
> >  arch/Kconfig                   |  39 ++++++++
> >  include/linux/compiler-clang.h |   2 +
> >  include/linux/compiler_types.h |   4 +
> >  include/linux/scs.h            |  88 ++++++++++++++++++
> >  init/init_task.c               |   6 ++
> >  init/main.c                    |   3 +
> >  kernel/Makefile                |   1 +
> >  kernel/fork.c                  |   9 ++
> >  kernel/sched/core.c            |   2 +
> >  kernel/sched/sched.h           |   1 +
> >  kernel/scs.c                   | 162 +++++++++++++++++++++++++++++++++
> >  12 files changed, 323 insertions(+)
> >  create mode 100644 include/linux/scs.h
> >  create mode 100644 kernel/scs.c
> > 
> > diff --git a/Makefile b/Makefile
> > index ffd7a912fc46..e401fa500f62 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -846,6 +846,12 @@ ifdef CONFIG_LIVEPATCH
> >  KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
> >  endif
> >  
> > +ifdef CONFIG_SHADOW_CALL_STACK
> > +KBUILD_CFLAGS	+= -fsanitize=shadow-call-stack
> > +DISABLE_SCS	:= -fno-sanitize=shadow-call-stack
> > +export DISABLE_SCS
> > +endif
> 
> I think it would be preferable to follow the example of CC_FLAGS_FTRACE
> so that this can be filtered out, e.g.
> 
> ifdef CONFIG_SHADOW_CALL_STACK
> CFLAGS_SCS := -fsanitize=shadow-call-stack
  ^^^ was this meant to be CC_FLAGS_SCS here

> KBUILD_CFLAGS += $(CFLAGS_SCS)
                     ^^^ and here?

> export CC_FLAGS_SCS
> endif
> 
> ... with removal being:
> 
> CFLAGS_REMOVE := $(CC_FLAGS_SCS)
> 
> ... or:
> 
> CFLAGS_REMOVE_obj.o := $(CC_FLAGS_SCS)
> 
> That way you only need to define the flags once, so the enable and
> disable falgs remain in sync by construction.
> 
> [...]

-- 
Kees Cook