Re: [PATCH v3] ubsan: Reintroduce signed overflow sanitizer

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 05, 2024 at 01:54:24PM +0100, Andrey Ryabinin wrote:
> 
> 
> On 2/5/24 10:37, Kees Cook wrote:
> 
> > ---
> >  include/linux/compiler_types.h |  9 ++++-
> >  lib/Kconfig.ubsan              | 14 +++++++
> >  lib/test_ubsan.c               | 37 ++++++++++++++++++
> >  lib/ubsan.c                    | 68 ++++++++++++++++++++++++++++++++++
> >  lib/ubsan.h                    |  4 ++
> >  scripts/Makefile.lib           |  3 ++
> >  scripts/Makefile.ubsan         |  3 ++
> >  7 files changed, 137 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > index 6f1ca49306d2..ee9d272008a5 100644
> > --- a/include/linux/compiler_types.h
> > +++ b/include/linux/compiler_types.h
> > @@ -282,11 +282,18 @@ struct ftrace_likely_data {
> >  #define __no_sanitize_or_inline __always_inline
> >  #endif
> >  
> > +/* Do not trap wrapping arithmetic within an annotated function. */
> > +#ifdef CONFIG_UBSAN_SIGNED_WRAP
> > +# define __signed_wrap __attribute__((no_sanitize("signed-integer-overflow")))
> > +#else
> > +# define __signed_wrap
> > +#endif
> > +
> >  /* Section for code which can't be instrumented at all */
> >  #define __noinstr_section(section)					\
> >  	noinline notrace __attribute((__section__(section)))		\
> >  	__no_kcsan __no_sanitize_address __no_profile __no_sanitize_coverage \
> > -	__no_sanitize_memory
> > +	__no_sanitize_memory __signed_wrap
> >  
> 
> Given this disables all kinds of code instrumentations,
> shouldn't we just add __no_sanitize_undefined here?

Yeah, that's a very good point.

> I suspect that ubsan's instrumentation usually doesn't cause problems
> because it calls __ubsan_* functions with all heavy stuff (printk, locks etc)
> only if code has an UB. So the answer to the question above depends on
> whether we want to ignore UBs in "noinstr" code or to get some weird side effect,
> possibly without proper UBSAN report in dmesg.

I think my preference would be to fail safe (i.e. leave in the
instrumentation), but the intent of noinstr is pretty clear. :P I wonder
if, instead, we could adjust objtool to yell about cases where calls are
made in noinstr functions (like it does for UACCESS)... maybe it already
does?

-Kees

-- 
Kees Cook