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

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Date:	Tue, 24 Jun 2014 12:08:16 -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 11:37 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/24, Kees Cook wrote:
>>
>> On Tue, Jun 24, 2014 at 10:27 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) {
>> >> +             pid_t failed;
>> >> +
>> >> +             if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
>> >> +                 (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
>> >> +                  is_ancestor(thread->seccomp.filter,
>> >> +                              caller->seccomp.filter)))
>> >> +                     continue;
>> >> +
>> >> +             /* Return the first thread that cannot be synchronized. */
>> >> +             failed = task_pid_vnr(thread);
>> >> +             /* If the pid cannot be resolved, then return -ESRCH */
>> >> +             if (failed == 0)
>> >> +                     failed = -ESRCH;
>> >
>> > forgot to mention, task_pid_vnr() can't fail. sighand->siglock is held,
>> > for_each_thread() can't see a thread which has passed unhash_process().
>>
>> Certainly good to know, but I'd be much more comfortable leaving this
>> check as-is. Having "failed" return with "0" would be very very bad
>> (userspace would think the filter had been successfully applied, etc).
>> I'd rather stay highly defensive here.
>
> OK, agreed. Although in this case I'd suggest
>
>                 if (WARN_ON(failed == 0))
>                         failed = -ESRCH;
>
> but I won't insist.

Excellent idea, yes! I'll include this as well.

-Kees

-- 
Kees Cook
Chrome OS Security
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