Re: [PATCH v7 1/8] overflow: Move and add few utility macros into overflow

Kees Cook
2022-08-22T20:12:40Z

On Tue, Aug 23, 2022 at 04:32:10AM +0900, Gwan-gyeong Mun wrote:
> On 8/22/22 11:05 PM, Andrzej Hajda wrote:
> > On 18.08.2022 02:12, Kees Cook wrote:
> > > On Thu, Aug 18, 2022 at 01:07:29AM +0200, Andi Shyti wrote:
> > > > [...]
> > > > > +#define safe_conversion(ptr, value) ({ \
> > > > > +    typeof(value) __v = (value); \
> > > > > +    typeof(ptr) __ptr = (ptr); \
> > > > > +    overflows_type(__v, *__ptr) ? 0 : ((*__ptr =
> > > > > (typeof(*__ptr))__v), 1); \
> > > > > +})
> > >
> > > I try to avoid "safe" as an adjective for interface names, since it
> > > doesn't really answer "safe from what?" This looks more like "assign, but
> > > zero when out of bounds". And it can be built from existing macros here:
> > >
> > >     if (check_add_overflow(0, value, ptr))
> > >         *ptr = 0;
> > >
> > > I actually want to push back on this a bit, because there can still be
> > > logic bugs built around this kind of primitive. Shouldn't out-of-bounds
> > > assignments be seen as a direct failure? I would think this would be
> > > sufficient:
> > >
> > > #define check_assign(value, ptr)    check_add_overflow(0, value, ptr)
> > >
> > > And callers would do:
> > >
> > >     if (check_assign(value, &var))
> > >         return -EINVAL;
> > >
> Yes, I also like check_assign() you suggested more than safe_conversion.
> As shown below, it would be more readable to return true when assign
> succeeds and false when it fails. What do you think?

No, this inverts the style of all the other check_*() functions, so it
should remain "non-zero is failure".

> /**
> * check_assign - perform a type conversion (cast) of an source value into
> * a new variable, checking that the destination is large enough to hold the
> * source value.
> *
> * @value: Source value
> * @ptr: Destination pointer address, If the pointer type is not used, a
> warning message is output during build.
> *
> * Returns:
> * If the value would overflow the destination, it returns false. If not
> return true.
> */
> #define check_assign(value, ptr) __must_check_overflow(({ \
> typecheck_pointer(ptr); \
> !__builtin_add_overflow(0, value, ptr); \
> }))

Please don't use the __builtin*s, instead stick to the check_* family,
as they correctly wrap the builtins and perform type checking, etc. As
mentioned, check_assign() should just be:

#define check_assign(value, ptr)    check_add_overflow(0, value, ptr)

I don't think any of the other code is needed? What's the use-case for
the other stuff? i.e. Why does anything need overflows_type()?

-Kees

--
Kees Cook