linux-kernel - Re: [PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG

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Date:	Tue, 24 Jun 2014 11:19:27 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Will Drewry <wad@...omium.org>,
	Julien Tinnes <jln@...omium.org>,
	David Drysdale <drysdale@...gle.com>,
	Linux API <linux-api@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-mips@...ux-mips.org,
	linux-arch <linux-arch@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

On Tue, Jun 24, 2014 at 10:08 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/23, Kees Cook wrote:
>>
>> +static pid_t seccomp_can_sync_threads(void)
>> +{
>> +     struct task_struct *thread, *caller;
>> +
>> +     BUG_ON(write_can_lock(&tasklist_lock));
>> +     BUG_ON(!spin_is_locked(&current->sighand->siglock));
>> +
>> +     if (current->seccomp.mode != SECCOMP_MODE_FILTER)
>> +             return -EACCES;
>> +
>> +     /* Validate all threads being eligible for synchronization. */
>> +     thread = caller = current;
>> +     for_each_thread(caller, thread) {
>
> You only need to initialize "caller" for for_each_thread(). Same for
> seccomp_sync_threads().

Thanks, I'll fix this up.

>> @@ -586,6 +701,17 @@ static long seccomp_set_mode_filter(unsigned int flags,
>>       if (IS_ERR(prepared))
>>               return PTR_ERR(prepared);
>>
>> +     /*
>> +      * If we're doing thread sync, we must hold tasklist_lock
>> +      * to make sure seccomp filter changes are stable on threads
>> +      * entering or leaving the task list. And we must take it
>> +      * before the sighand lock to avoid deadlocking.
>> +      */
>> +     if (flags & SECCOMP_FILTER_FLAG_TSYNC)
>> +             write_lock_irqsave(&tasklist_lock, taskflags);
>> +     else
>> +             __acquire(&tasklist_lock); /* keep sparse happy */
>> +
>
> Why? ->siglock should be enough, it seems.
>
> It obviously does not protect the global process list, but *sync_threads()
> only care about current's thread group list, no?

I think I was concerned about the exit case, but reading through those
paths again, I can't find a race. Calls to put_seccomp_filter() should
already be safe. Let me see what happens if I drop the tasklist_lock
usage...

-Kees


-- 
Kees Cook
Chrome OS Security
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