Re: [PATCH v3 02/11] selftests:lib:test_counters: add new test for counters

[Kees Cook]

On Fri, Oct 09, 2020 at 09:55:57AM -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. 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.
> 
> Signed-off-by: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook