linux-kernel - Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG

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Message-ID: <CAGXu5jLh-a2x0N+kHW7qbZSW0gOPs8v05EzBFkMYQwhft8o2aA@mail.gmail.com>
Date:	Wed, 25 Jun 2014 11:31:48 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-arch <linux-arch@...r.kernel.org>, linux-mips@...ux-mips.org,
	Will Drewry <wad@...omium.org>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Julien Tinnes <jln@...omium.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Drysdale <drysdale@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Alexei Starovoitov <ast@...mgrid.com>
Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

On Wed, Jun 25, 2014 at 11:20 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/25, Kees Cook wrote:
>>
>> On Wed, Jun 25, 2014 at 10:24 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >
>> > However, do_execve() takes cred_guard_mutex at the start in prepare_bprm_creds()
>> > and drops it in install_exec_creds(), so it should solve the problem?
>>
>> I can't tell yet. I'm still trying to understand the order of
>> operations here. It looks like de_thread() takes the sighand lock.
>> do_execve_common does:
>>
>> prepare_bprm_creds (takes cred_guard_mutex)
>> check_unsafe_exec (checks nnp to set LSM_UNSAFE_NO_NEW_PRIVS)
>> prepare_binprm (handles suid escalation, checks nnp separately)
>>     security_bprm_set_creds (checks LSM_UNSAFE_NO_NEW_PRIVS)
>> exec_binprm
>>     load_elf_binary
>>         flush_old_exec
>>             de_thread (takes and releases sighand->lock)
>>         install_exec_creds (releases cred_guard_mutex)
>
> Yes, and note that when cred_guard_mutex is dropped all other threads
> are already killed,
>
>> I don't see a way to use cred_guard_mutex during tsync (which holds
>> sighand->lock) without dead-locking. What were you considering here?
>
> Just take/drop current->signal->cred_guard_mutex along with ->siglock
> in seccomp_set_mode_filter() ? Unconditionally on depending on
> SECCOMP_FILTER_FLAG_TSYNC.

Yeah, this looks good. *whew* Testing it now, so far so good.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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