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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 19:00:39 +0200
Cc: LKML <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 <linux-api@vger.kernel.org>, "x86@kernel.org" <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-mips@linux-mips.org, linux-arch <linux-arch@vger.kernel.org>, linux-security-module <linux-security-module@vger.kernel.org>
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On 06/25, Kees Cook wrote:
>
> On Wed, Jun 25, 2014 at 6:51 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> > 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.
>
> Hm, actually, now I'm worried about smp_load_acquire() being too slow
> in run_filters().
>
> The ordering must be:
> - task->seccomp.filter must be valid before
> - task->seccomp.mode is set, which must be valid before
> - TIF_SECCOMP is set
>
> But I don't want to impact secure_computing(). What's the best way to
> make sure this ordering is respected?

Cough, confused... can't understand even after I read the email from Andy.

We do not care if __secure_computing() misses the recently added filter,
this can happen anyway, whatever we do.

seccomp.mode is frozen after we set it != SECCOMP_MODE_DISABLED.

So we should only worry if set_tsk_thread_flag(TIF_SECCOMP) actually
changes this bit and makes __secure_computing() possible. If we add
smp_mb__before_atomic() into seccomp_assign_mode() and rmb() at the
start of __secure_computing() everything should be fine?

Oleg.


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