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Date:   Fri, 16 Dec 2022 15:05:13 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Jann Horn <jannh@google.com>,
        Seth Jenkins <sethjenkins@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Eric Biggers <ebiggers@google.com>,
        Huang Ying <ying.huang@intel.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-doc@vger.kernel.org,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Joel Savitz <jsavitz@redhat.com>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Rob Herring <robh@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] exit: Allow oops_limit to be disabled
Message-ID: <Y5x7GXeluFmZ8E0E@hirez.programming.kicks-ass.net>
References: <20221202210617.never.105-kees@kernel.org>
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On Fri, Dec 02, 2022 at 01:06:21PM -0800, Kees Cook wrote:

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -954,7 +954,7 @@ void __noreturn make_task_dead(int signr)
>  	 * To make sure this can't happen, place an upper bound on how often the
>  	 * kernel may oops without panic().
>  	 */
> -	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit))
> +	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit) && oops_limit)
>  		panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit);
>  

That's dodgy, please write as:

	limit = READ_ONCE(oops_limit);
	if (atomic_inc_return(&oops_count) >= limit && limit)

So we don't explicitly add a reload that negates the whole READ_ONCE().