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Re: [PATCH v8 5/9] seccomp: split mode set routines

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines
From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 25 Jun 2014 15:51:22 +0200
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Alexei Starovoitov <ast@plumgrid.com>, Andrew Morton <akpm@linux-foundation.org>, Daniel Borkmann <dborkman@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, linux-api@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org
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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.


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