Re: [PATCH 1/2] overflow: Implement size_t saturating arithmetic helpers

[Kees Cook <keescook@chromium.org>]

*thread necromancy*

On Tue, Sep 21, 2021 at 08:51:53AM +0200, Rasmus Villemoes wrote:
> Not that I can see that the __must_check matters much for these anyway;
> if anybody does
> 
>   size_mul(foo, bar);
> 
> that's just a statement with no side effects, so probably the compiler
> would warn anyway, or at least nobody can then go on to do anything
> "wrong". Unlike the check_*_overflow(), which have the (possibly
> wrapped) result in a output-pointer and the "did it overflow" as the
> return value, so you can do
> 
>   check_mul_overflow(a, b, &d);
>   do_stuff_with(d);
> 
> were it not for the __must_check wrapper.
> 
> [Reminder: __must_check is a bit of a misnomer, the attribute is really
> warn_unused_result, and there's no requirement that the result is part
> of the controlling expression of an if() or while() - just passing the
> result on directly to some other function counts as a "use", which is
> indeed what we do with the size wrappers.]

What I'd really like is a "store this in a size_t" check to catch dumb
storage size problems (or related overflows). In other words:

size_t big1 = 2147483647;
size_t big2 = 2147483647;

/* Doesn't overflow, but 4611686014132420609 becomes a 1 for int */
int size = size_mul(big1, big2);
...
ptr = kmalloc(size, GFP_KERNEL); /* Allocates a 1 instead... */

I could solve this but removing the assignment, but then I can't compose
calls:

static inline size_t __size_mul(size_t f1, size_t f2)
{
	size_t out;
	if (check_mul_overflow(f1, f2, &out))
		out = SIZE_MAX;
	return out;
}

#define size_mul(f1, f2, out) do { \
	BUILD_BUG_ON(!__same_type(out, size_t)); \
	out = __size_mul(f1, f2); \
} while (0)

i.e. now I can't do size_mul(size_add(...), size_add(...))

Better would be to build the entire kernel with -Wconversion. :)

-- 
Kees Cook