linux-kernel - Re: [PATCH] seccomp: do not reject initial filter using TSYNC

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Date:	Thu, 17 Jul 2014 10:59:19 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	Oleg Nesterov <oleg@...hat.com>,
	David Drysdale <drysdale@...gle.com>
Subject: Re: [PATCH] seccomp: do not reject initial filter using TSYNC

On Thu, Jul 17, 2014 at 10:56 AM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 17, 2014 at 10:53 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, Jul 17, 2014 at 10:49 AM, Kees Cook <keescook@...omium.org> wrote:
>>> There was an unneeded sanity check in the TSYNC code that was causing
>>> the first filter applied to not allow the TSYNC flag. Additionally,
>>> this optimizes the thread loops to skip "current". It was harmless, but
>>> better to not cause problems in the future.
>>>
>>> Reported-by: David Drysdale <drysdale@...gle.com>
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>> ---
>>> This goes on top of the v11 seccomp-tsync series. If I should respin
>>> as v12, please let me know.
>>>
>>> Thanks!
>>> ---
>>>  kernel/seccomp.c |   11 ++++++++---
>>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>>> index 2125b83ccfd4..0e0c6905b81d 100644
>>> --- a/kernel/seccomp.c
>>> +++ b/kernel/seccomp.c
>>> @@ -255,14 +255,15 @@ static inline pid_t seccomp_can_sync_threads(void)
>>>         BUG_ON(!mutex_is_locked(&current->signal->cred_guard_mutex));
>>>         BUG_ON(!spin_is_locked(&current->sighand->siglock));
>>>
>>> -       if (current->seccomp.mode != SECCOMP_MODE_FILTER)
>>> -               return -EACCES;
>>> -
>>>         /* Validate all threads being eligible for synchronization. */
>>>         caller = current;
>>>         for_each_thread(caller, thread) {
>>>                 pid_t failed;
>>>
>>> +               /* Skip current, since it is initiating the sync. */
>>> +               if (thread == current)
>>> +                       continue;
>>> +
>>
>> Should that be "thread == caller"?
>
> caller shouldn't be changing, correct? Won't it be the same?
>

I assumed that you loaded caller once as an optimization -- ISTR that,
at least at some point, accessing current was a slightly expensive.
Maybe this is moot now.

Anyway, the rest of the code in there is comparing thread to caller,
using caller seems a bit more consistent.

--Andy

> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC
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