Am Montag, dem 23.10.2023 um 12:52 -0700 schrieb Kees Cook:
> On Fri, Oct 20, 2023 at 09:54:05PM +0200, Martin Uecker wrote:
> > Am Freitag, dem 20.10.2023 um 18:48 +0000 schrieb Qing Zhao:
> > >
> > > > On Oct 20, 2023, at 2:34 PM, Kees Cook <keesc...@chromium.org> wrote:
> > > >
> > > > On Fri, Oct 20, 2023 at 11:50:11AM +0200, Martin Uecker wrote:
> > > > > Am Donnerstag, dem 19.10.2023 um 16:33 -0700 schrieb Kees Cook:
> > > > > > On Wed, Oct 18, 2023 at 09:11:43PM +0000, Qing Zhao wrote:
> > > > > > > As I replied to Martin in another email, I plan to do the
> > > > > > > following to resolve this issue:
> > > > > > >
> > > > > > > 1. No specification for signed or unsigned for counted_by field.
> > > > > > > 2. Add a sanitizer option -fsanitize=counted-by-bound to catch
> > > > > > > the cases when the size of the counted-by is not positive.
> > > > > >
> > > > > > I don't understand why this needs to be a runtime sanitizer. The
> > > > > > signedness is known at compile time, so I would expect a -W option.
> > > > >
> > > > > The signedness of the type but not of the value.
> > > > >
> > > > > But I would not want to have a warning for signed
> > > > > counter types by default because I would prefer
> > > > > to use signed types (for various reasons including
> > > > > better overflow detection).
> > > > >
> > > > > > Or
> > > > > > do you mean you'd split up -fsanitize=bounds between unsigned and
> > > > > > signed
> > > > > > indexes? I'd find that kind of awkward for the kernel... but I feel
> > > > > > like
> > > > > > I've misunderstood something. :)
> > > > > >
> > > > > > -Kees
> > > > >
> > > > > The idea would be to detect at run-time the case
> > > > > if x->buf is used at a time where x->counter
> > > > > is negative and also when x->counter * sizeof(x->buf[0])
> > > > > overflows or is too big.
> > > > >
> > > > > This would be similar to
> > > > >
> > > > > int a[n];
> > > > >
> > > > > where it is detected at run-time if n is not-positive.
> > > >
> > > > Right. I guess what I mean to say is that I would expect this case to
> > > > already be caught by -fsanitize=bounds -- I don't see a reason to add an
> > > > additional sanitizer option.
> > > >
> > > > struct foo {
> > > > int count;
> > > > int array[] __counted_by(count);
> > > > };
> > > >
> > > > foo->count = 5;
> > > > foo->array[0] = 1; // ok
> > > > foo->array[10] = 1; // -fsanitize=bounds will catch this
> > > > foo->array[-10] = 1; // -fsanitize=bounds will catch this too
> > > >
> > > >
> > >
> > > just checked this testing case with my GCC, and YES, -fsanitize=bounds
> > > indeed caught this error:
> > >
> > > ttt_1.c:31:12: runtime error: index 10 out of bounds for type 'char [*]'
> > > ttt_1.c:32:12: runtime error: index -10 out of bounds for type 'char [*]’
> > >
> >
> > Yes, but I thought we were discussing the case where count is
> > set to a negative value:
> >
> > foo->count = -1;
> > int x = foo->array[3]; // UBSan should diagnose this
>
> Oh right, I keep thinking about it backwards.
>
> Yeah, we can't trap the "count" assignment, because it may be getting used
> for other purposes. But yeah, access to "array" should trap if "count"
> is negative.
>
> > And also the case when foo->array becomes too big.
>
> How do you mean?
count * sizeof(member) could overflow or otherwise be
bigger than allowed.
Martin
