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Date:   Wed, 25 Jun 2014 15:51:22 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
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        linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org,
        linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines
Message-ID: <20140625135121.GB7892@redhat.com>
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On 06/24, Kees Cook wrote:
>
> +static inline void seccomp_assign_mode(struct task_struct *task,
> +				       unsigned long seccomp_mode)
> +{
> +	BUG_ON(!spin_is_locked(&task->sighand->siglock));
> +
> +	task->seccomp.mode = seccomp_mode;
> +	set_tsk_thread_flag(task, TIF_SECCOMP);
> +}

OK, but unless task == current this can race with secure_computing().
I think this needs smp_mb__before_atomic() and secure_computing() needs
rmb() after test_bit(TIF_SECCOMP).

Otherwise, can't __secure_computing() hit BUG() if it sees the old
mode == SECCOMP_MODE_DISABLED ?

Or seccomp_run_filters() can see ->filters == NULL and WARN(),
smp_load_acquire() only serializes that LOAD with the subsequent memory
operations.

Oleg.