From: Kees Cook <keescook@chromium.org>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: "Gustavo A . R . Silva" <gustavoars@kernel.org>,
Nathan Chancellor <nathan@kernel.org>,
Jason Gunthorpe <jgg@ziepe.ca>,
Nick Desaulniers <ndesaulniers@google.com>,
Leon Romanovsky <leon@kernel.org>,
Keith Busch <kbusch@kernel.org>, Len Baker <len.baker@gmx.com>,
linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH 1/2] overflow: Implement size_t saturating arithmetic helpers
Date: Mon, 24 Jan 2022 13:13:20 -0800 [thread overview]
Message-ID: <202201241237.C82267B66C@keescook> (raw)
In-Reply-To: <aa42ebfa-03b8-93fa-e036-a7507397d0dc@rasmusvillemoes.dk>
*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
next prev parent reply other threads:[~2022-01-24 23:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-20 18:08 [PATCH 0/2] overflow: Implement size_t saturating arithmetic helpers Kees Cook
2021-09-20 18:08 ` [PATCH 1/2] " Kees Cook
2021-09-20 22:06 ` Nick Desaulniers
2021-09-21 1:38 ` Kees Cook
2021-09-21 6:51 ` Rasmus Villemoes
2021-09-21 19:07 ` Kees Cook
2022-01-24 21:13 ` Kees Cook [this message]
2022-01-24 21:16 ` Nick Desaulniers
2022-01-25 12:58 ` Jason Gunthorpe
2021-09-20 18:08 ` [PATCH 2/2] test_overflow: Regularize test reporting output Kees Cook
2021-09-20 22:10 ` Nick Desaulniers
2021-09-21 6:56 ` Rasmus Villemoes
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