linux-hardening - Re: [RFC] Mitigating unexpected arithmetic overflow

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Date: Thu, 16 May 2024 13:07:09 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Kees Cook <kees@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mark Rutland <mark.rutland@....com>,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [RFC] Mitigating unexpected arithmetic overflow

On Thu, May 16, 2024 at 12:48:47PM -0700, Justin Stitt wrote:
> I don't think we're capable of identifying every single problematic
> overflow/wraparound case in the kernel, this is pretty obvious
> considering we've had decades to do so. Instead, it seems much more
> feasible that we annotate (very, very minimally so as not to disrupt
> code readability and style) the spots where we _know_ overflow should
> happen.

For the baby steps Linus wants, we can walk this path:

- Finish the *signed* integer overflow refactoring/annotation.
  This is nearly done already, and every case we've found is either
  a legitimate bug (thankfully rare), or happens in code that is
  either accidentally correct (thanks to no UB), or the correctness is
  very unclear. Refactoring these cases improves readability for
  everyone and doesn't change the behavior.

- Begin *signed* integer implicit truncation refactoring/annotation.
  As Linus suggested, dealing with this will catch a bunch of the flaws
  we've seen recently. Handling the false positives here will need some
  investigation and some compiler support, and that'll happen in
  parallel.

- Tackle *unsigned* integer overflow on a per-type basis: we can start
  with the place Linus called out: size_t. This will let us focus on the
  first of the unsigned types that is not commonly wrapping, and is a
  regular place that unexpected overflow gets the kernel into big
  trouble.

What we learn from these three steps should inform us what further steps
down this path can look like.

-Kees

-- 
Kees Cook