Re: [PATCH 02/82] overflow: Introduce add_would_overflow()

[Kees Cook]

On Tue, Jan 23, 2024 at 09:03:10AM +0100, Rasmus Villemoes wrote:
> On 23/01/2024 01.26, Kees Cook wrote:
> > For instances where only the overflow needs to be checked (and the sum
> > isn't used), provide the new helper add_would_overflow(), which is
> > a wrapper for check_add_overflow().
> > 
> > Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> > Cc: linux-hardening@xxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  include/linux/overflow.h | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> > index 099f2e559aa8..ac088f73e0fd 100644
> > --- a/include/linux/overflow.h
> > +++ b/include/linux/overflow.h
> > @@ -108,6 +108,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
> >  		__builtin_add_overflow(__filter_integral(a), b,		\
> >  				       __filter_ptrint(d))))
> >  
> > +/**
> > + * add_would_overflow() - Check if an addition would overflow
> > + * @a: first addend
> > + * @b: second addend
> > + *
> > + * Returns true if the sum would overflow.
> > + *
> > + * To keep a copy of the sum when the addition doesn't overflow, use
> > + * check_add_overflow() instead.
> > + */
> > +#define add_would_overflow(a, b)			\
> > +	__must_check_overflow(({			\
> > +		size_t __result;			\
> > +		check_add_overflow(a, b, &__result);\
> > +	}))
> 
> Hm, I think this is a bit too ill-defined. Why is the target type
> hard-coded as size_t? What if a and b are u64, and we're on a 32 bit
> target? Then a+b might not overflow but this helper would claim it did.

Oooh, yes. That's no good. Thanks.

> But we also cannot just use typeof(a+b) instead of size_t, since that
> breaks when a and b are narrower than int (adding two u16 never
> overflows since they get promoted to int, but then if assigning the
> result to a u16 one truncates...).

The add_would_overflow() is aimed at replacing the "v + o < v" pattern,
so perhaps use typeof(a) ?

> Perhaps the target type must be explicit? sum_fits_in_type(T, a, b) ?
> IDK, I just don't think size_t is the right thing to use in something
> that is otherwise supposed to be type-generic.

I will use typeof(a) and check binary differences to see if there are
any places doing something unexpected...

-Kees

-- 
Kees Cook