linux-kernel - Re: [PATCH v2 02/11] selftests:lib:test

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Date:   Wed, 7 Oct 2020 11:12:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     shuah@...nel.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 02/11] selftests:lib:test_counters: add new test for
 counters

On Tue, Oct 06, 2020 at 02:44:33PM -0600, Shuah Khan wrote:
> Add a new selftest for testing counter_atomic* Counters API. This test
> load test_counters test modules and unloads.
> 
> The test module runs tests and prints results in dmesg.
> 
> 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. Counter will wrap
> around to 0 when it overflows and should not be used to guard resource
> lifetimes, device usage and open counts that control state changes, and
> pm states.
> 
> 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.
> 
> Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>

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

-- 
Kees Cook