From: Kees Cook <keescook@chromium.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Oleg Nesterov <oleg@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
"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>
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines
Date: Fri, 27 Jun 2014 11:52:31 -0700 [thread overview]
Message-ID: <CAGXu5j+wPvAuo6pVc_XoJqv7CvW6yDf3YE6P7tFwtO9hfcUUsg@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUPxTxseJ=sOhD9CyJPtOqCR5sL8yx7KezLmLZcFSFNMA@mail.gmail.com>
On Fri, Jun 27, 2014 at 11:39 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Fri, Jun 27, 2014 at 11:33 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, Jun 25, 2014 at 11:07 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> On Wed, Jun 25, 2014 at 11:00 AM, Kees Cook <keescook@chromium.org> wrote:
>>>> On Wed, Jun 25, 2014 at 10:51 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>>>> On 06/25, Andy Lutomirski wrote:
>>>>>>
>>>>>> On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>>>>> > On 06/25, Andy Lutomirski wrote:
>>>>>> >>
>>>>>> >> Write the filter, then smp_mb (or maybe a weaker barrier is okay),
>>>>>> >> then set the bit.
>>>>>> >
>>>>>> > Yes, exactly, this is what I meant. Plas rmb() in __secure_computing().
>>>>>> >
>>>>>> > But I still can't understand the rest of your discussion about the
>>>>>> > ordering we need ;)
>>>>>>
>>>>>> Let me try again from scratch.
>>>>>>
>>>>>> Currently there are three relevant variables: TIF_SECCOMP,
>>>>>> seccomp.mode, and seccomp.filter. __secure_computing needs
>>>>>> seccomp.mode and seccomp.filter to be in sync, and it wants (but
>>>>>> doesn't really need) TIF_SECCOMP to be in sync as well.
>>>>>>
>>>>>> My suggestion is to rearrange it a bit. Move mode into seccomp.filter
>>>>>> (so that filter == NULL implies no seccomp) and don't check
>>>>
>>>> This would require that we reimplement mode 1 seccomp via mode 2
>>>> filters. Which isn't too hard, but may add complexity.
>>>>
>>>>>> TIF_SECCOMP in secure_computing. Then turning on seccomp is entirely
>>>>>> atomic except for the fact that the seccomp hooks won't be called if
>>>>>> filter != NULL but !TIF_SECCOMP. This removes all ordering
>>>>>> requirements.
>>>>>
>>>>> Ah, got it, thanks. Perhaps I missed somehing, but to me this looks like
>>>>> unnecessary complication at first glance.
>>>>>
>>>>> We alredy have TIF_SECCOMP, we need it anyway, and we should only care
>>>>> about the case when this bit is actually set, so that we can race with
>>>>> the 1st call of __secure_computing().
>>>>>
>>>>> Otherwise we are fine: we can miss the new filter anyway, ->mode can't
>>>>> be changed it is already nonzero.
>>>>>
>>>>>> Alternatively, __secure_computing could still BUG_ON(!seccomp.filter).
>>>>>> In that case, filter needs to be set before TIF_SECCOMP is set, but
>>>>>> that's straightforward.
>>>>>
>>>>> Yep. And this is how seccomp_assign_mode() already works? It is called
>>>>> after we change ->filter chain, it changes ->mode before set(TIF_SECCOMP)
>>>>> just it lacks a barrier.
>>>>
>>>> Right, I think the best solution is to add the barrier. I was
>>>> concerned that adding the read barrier in secure_computing would have
>>>> a performance impact, though.
>>>>
>>>
>>> I can't speak for ARM, but I think that all of the read barriers are
>>> essentially free on x86. (smp_mb is a very different story, but that
>>> shouldn't be needed here.)
>>
>> It looks like SMP ARM issues dsb for rmb, which seems a bit expensive.
>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204g/CIHJFGFE.html
>>
>> If I skip the rmb in the secure_computing call before checking mode,
>> it sounds like I run the risk of racing an out-of-order TIF_SECCOMP vs
>> mode and filter. This seems unlikely to me, given an addition of the
>> smp_mb__before_atomic() during the seccomp_assign_mode()? I guess I
>> don't have a sense of how aggressively ARM might do data caching in
>> this area. Could the other thread actually see TIF_SECCOMP get set but
>> still have an out of date copy of seccomp.mode?
>>
>> I really want to avoid adding anything to the secure_computing()
>> execution path. :(
>
> Hence my suggestion to make the ordering not matter. No ordering
> requirement, no barriers.
I may be misunderstanding something, but I think there's still an
ordering problem. We'll have TIF_SECCOMP already, so if we enter
secure_computing with a NULL filter, we'll kill the process.
Merging .mode and .filter would remove one of the race failure paths:
having TIF_SECCOMP and not having a mode set (leading to BUG). With
the merge, we could still race and land in the same place as have
TIF_SECCOMP and mode==2, but filter==NULL, leading to WARN and kill.
I guess the question is how large is the race risk on ARM? Is it
possible to have TIF_SECCOMP that far out of sync for the thread?
-Kees
--
Kees Cook
Chrome OS Security
next prev parent reply other threads:[~2014-06-27 18:52 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-24 20:48 [PATCH v8 0/9] seccomp: add thread sync ability Kees Cook
2014-06-24 20:48 ` [PATCH v8 1/9] seccomp: create internal mode-setting function Kees Cook
2014-06-24 20:48 ` [PATCH v8 2/9] seccomp: split filter prep from check and apply Kees Cook
2014-06-24 20:48 ` [PATCH v8 3/9] seccomp: introduce writer locking Kees Cook
2014-06-25 14:03 ` Oleg Nesterov
2014-06-25 18:07 ` Oleg Nesterov
2014-06-25 18:29 ` Oleg Nesterov
2014-06-27 17:27 ` Kees Cook
2014-06-24 20:48 ` [PATCH v8 4/9] sched: move no_new_privs into new atomic flags Kees Cook
2014-06-25 13:43 ` Oleg Nesterov
2014-06-25 14:44 ` Kees Cook
2014-06-24 20:48 ` [PATCH v8 5/9] seccomp: split mode set routines Kees Cook
2014-06-25 13:51 ` Oleg Nesterov
2014-06-25 14:51 ` Kees Cook
2014-06-25 16:10 ` Andy Lutomirski
2014-06-25 16:54 ` Kees Cook
2014-06-25 17:03 ` Andy Lutomirski
2014-06-25 17:32 ` Oleg Nesterov
2014-06-25 17:38 ` Andy Lutomirski
2014-06-25 17:51 ` Oleg Nesterov
2014-06-25 18:00 ` Kees Cook
2014-06-25 18:07 ` Andy Lutomirski
2014-06-27 18:33 ` Kees Cook
2014-06-27 18:39 ` Andy Lutomirski
2014-06-27 18:52 ` Kees Cook [this message]
2014-06-27 18:56 ` Andy Lutomirski
2014-06-27 19:04 ` Kees Cook
2014-06-27 19:11 ` Andy Lutomirski
2014-06-27 19:27 ` Oleg Nesterov
2014-06-27 19:31 ` Andy Lutomirski
2014-06-27 19:55 ` Oleg Nesterov
2014-06-27 20:08 ` Andy Lutomirski
2014-06-27 20:56 ` Kees Cook
2014-06-25 17:00 ` Oleg Nesterov
2014-06-24 20:48 ` [PATCH v8 6/9] seccomp: add "seccomp" syscall Kees Cook
2014-06-24 20:48 ` [PATCH v8 7/9] ARM: add seccomp syscall Kees Cook
2014-06-24 20:48 ` [PATCH v8 8/9] MIPS: " Kees Cook
2014-06-24 20:48 ` [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC Kees Cook
2014-06-25 14:21 ` Oleg Nesterov
2014-06-25 15:08 ` Kees Cook
2014-06-25 16:52 ` Oleg Nesterov
2014-06-25 17:09 ` Kees Cook
2014-06-25 17:24 ` Oleg Nesterov
2014-06-25 17:40 ` Andy Lutomirski
2014-06-25 17:57 ` Kees Cook
2014-06-25 18:09 ` Andy Lutomirski
2014-06-25 18:25 ` Kees Cook
2014-06-25 18:20 ` Oleg Nesterov
2014-06-25 18:31 ` Kees Cook
2014-06-24 20:56 ` [PATCH v8 1/1] man-pages: seccomp.2: document syscall Kees Cook
2014-06-25 13:04 ` One Thousand Gnomes
2014-06-25 15:10 ` Kees Cook
2014-06-25 17:54 ` Michael Kerrisk (man-pages)
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