Re: [PATCH 06/14] Fix CFLAGS for UBSAN_BOUNDS on Clang

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 17, 2020 at 11:35:40AM +0000, George Popescu wrote:
> On Thu, Sep 17, 2020 at 08:37:07AM +0200, Marco Elver wrote:
> > So, it seems that local-bounds can still catch some rare OOB accesses,
> > where KASAN fails to catch it because the access might skip over the
> > redzone.
> > 
> > The other more interesting bit of history is that
> > -fsanitize=local-bounds used to be -fbounds-checking, and meant for
> > production use as a hardening feature:
> > http://lists.llvm.org/pipermail/llvm-dev/2012-May/049972.html
> > 
> > And local-bounds just does not behave like any other sanitizer as a
> > result, it just traps. The fact that it's enabled via
> > -fsanitize=local-bounds (or just bounds) but hasn't much changed in
> > behaviour is a little unfortunate.
> 
> > I suppose there are 3 options:
> > 
> > 1. George implements trap handling somehow. Is this feasible? If not,
> > why not? Maybe that should also have been explained in the commit
> > message.
> > 
> > 2. Only enable -fsanitize=local-bounds if UBSAN_TRAP was selected, at
> > least for as long as Clang traps for local-bounds. I think this makes
> > sense either way, because if we do not expect UBSAN to trap, it really
> > should not trap!
> > 
> > 3. Change the compiler. As always, this will take a while to implement
> > and then to reach whoever should have that updated compiler.
> > 
> > Preferences?
> Considering of what you said above, I find option 2 the most elegant.
> The first one doesn't sound doable for the moment, also the third.
> I will edit this patch considering your comments and resend it to the
> list.

I have a slightly different suggestion that is very nearly #2 above:
split local-bounds into a separate CONFIG that requires UBSAN_TRAP, and
then carefully document both:
- what does it catch that "bounds" doesn't
- why it only operates in trap mode

The rationale I have is that I don't like the coverage of some
mitigation or detection to "silently" vary between builds. e.g. someone
would build with/without UBSAN_TRAP and end up with unexpectedly
different coverage. I'd rather there be a separate CONFIG that appears.

-- 
Kees Cook