linux-kernel - Re: [PATCH v3 01/11] counters: Introduce counter

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Date:   Fri, 9 Oct 2020 11:00:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     corbet@....net, gregkh@...uxfoundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 01/11] counters: Introduce counter_atomic* counters

On Fri, Oct 09, 2020 at 09:55:56AM -0600, Shuah Khan wrote:
> Introduce Simple atomic counters.
> 
> There are a number of atomic_t usages in the kernel where atomic_t api
> is used strictly for counting and not for managing object lifetime. In
> some cases, atomic_t might not even be needed.
> 
> The purpose of these counters is to clearly differentiate atomic_t
> counters from atomic_t usages that guard object lifetimes, hence prone
> to overflow and underflow errors. It allows tools that scan for underflow
> and overflow on atomic_t usages to detect overflow and underflows to scan
> just the cases that are prone to errors.
> 
> Simple atomic counters api provides interfaces for simple atomic counters
> that just count, and don't guard resource lifetimes. The interfaces are
> built on top of atomic_t api, providing a smaller subset of atomic_t
> interfaces necessary to support simple counters.
> 
> Counter wraps around to INT_MIN when it overflows and should not be used
> to guard resource lifetimes, device usage and open counts that control
> state changes, and pm states. Overflowing to INT_MIN is consistent with
> the atomic_t api, which it is built on top of.
> 
> Using counter_atomic* to guard lifetimes could lead to use-after free
> when it overflows and undefined behavior when used to manage state
> changes and device usage/open states.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook