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

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Date: Fri, 17 May 2024 09:45:20 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Kees Cook <keescook@...omium.org>
Cc: Justin Stitt <justinstitt@...gle.com>,
 Peter Zijlstra <peterz@...radead.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

Am 5/8/2024 um 10:07 PM schrieb Linus Torvalds:
> And no, the answer is ABSOLUTELY NOT to add cognitive load on kernel
> developers by adding yet more random helper types and/or functions.

Just to show an option without "more types and helper functions", one 
could also instead add a coverage requirement:

Every arithmetic operation should either:
- have a test case where the wrap around happens, or
- have a static analyser say that overflow can not happen, or
- have a static analyser say that overflow is fine (e.g., your a+b < a case)

Then the answer to safe wrap situations isn't to make the kernel code 
less readable, but to have a component-level test that shows that the 
behavior on overflow (in at least one case :)) ) is what the developer 
expected.

For static analysis to prove that overflow can not happen, one sometimes 
would need to add BUG_ON() assertions to let the analyser know the 
assumptions on surrounding code, which has its own benefits.

static inline u32 __item_offset(u32 val)
{
         BUG_ON(val > INT_MAX / ITEM_SIZE_PER_UNIT);
         return val * ITEM_SIZE_PER_UNIT;
}

Obviously, the effort involved is still high. Maybe if someone as a pet 
project proves first that something in this direction is actually worth 
the effort (by uncovering a heap of bugs), one could offer this kind of 
check as an opt-in.

Best wishes,

   jonas