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 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
Date: Wed, 25 Jun 2014 11:25:26 -0700 [thread overview]
Message-ID: <CAGXu5j+Gw8V5qFueWJiA2rEntALFQ1KXOJaPg3OcJZKDCibyMg@mail.gmail.com> (raw)
In-Reply-To: <CALCETrXr+g6tM+9gQJKKnGj_XoyXi+7ZQpAWjykhOAErpE8MBw@mail.gmail.com>
On Wed, Jun 25, 2014 at 11:09 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Jun 25, 2014 at 10:57 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, Jun 25, 2014 at 10:24 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> On 06/25, Kees Cook wrote:
>>>>
>>>> On Wed, Jun 25, 2014 at 9:52 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>>> >
>>>> > Yes, at least this should close the race with suid-exec. And there are no
>>>> > other users. Except apparmor, and I hope you will check it because I simply
>>>> > do not know what it does ;)
>>>> >
>>>> >> I wonder if changes to nnp need to "flushed" during syscall entry
>>>> >> instead of getting updated externally/asynchronously? That way it
>>>> >> won't be out of sync with the seccomp mode/filters.
>>>> >>
>>>> >> Perhaps secure computing needs to check some (maybe seccomp-only)
>>>> >> atomic flags and flip on the "real" nnp if found?
>>>> >
>>>> > Not sure I understand you, could you clarify?
>>>>
>>>> Instead of having TSYNC change the nnp bit, it can set a new flag, say:
>>>>
>>>> task->seccomp.flags |= SECCOMP_NEEDS_NNP;
>>>>
>>>> This would be set along with seccomp.mode, seccomp.filter, and
>>>> TIF_SECCOMP. Then, during the next secure_computing() call that thread
>>>> makes, it would check the flag:
>>>>
>>>> if (task->seccomp.flags & SECCOMP_NEEDS_NNP)
>>>> task->nnp = 1;
>>>>
>>>> This means that nnp couldn't change in the middle of a running syscall.
>>>
>>> Aha, so you were worried about the same thing. Not sure we need this,
>>> but at least I understand you and...
>>>
>>>> Hmmm. Perhaps this doesn't solve anything, though? Perhaps my proposal
>>>> above would actually make things worse, since now we'd have a thread
>>>> with seccomp set up, and no nnp. If it was in the middle of exec,
>>>> we're still causing a problem.
>>>
>>> Yes ;)
>>>
>>>> I think we'd also need a way to either delay the seccomp changes, or
>>>> to notice this condition during exec. Bleh.
>>>
>>> Hmm. confused again,
>>
>> I mean to suggest that the tsync changes would be stored in each
>> thread, but somewhere other than the true seccomp struct, but with
>> TIF_SECCOMP set. When entering secure_computing(), current would check
>> for the "changes to sync", and apply them, then start the syscall. In
>> this way, we can never race a syscall (like exec).
>
> I'm not sure that helps. If you set a pending filter part-way through
> exec, and exec copies that pending filter but doesn't notice NNP, then
> there's an exploitable race.
>
>>
>>>> What actually happens with a multi-threaded process calls exec? I
>>>> assume all the other threads are destroyed?
>>>
>>> Yes. But this is the point-of-no-return, de_thread() is called after the execing
>>> thared has already passed (say) check_unsafe_exec().
>>>
>>> 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)
>>
>> 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?
>
> Grab cred_guard_mutex and then sighand->lock, perhaps?
Ah, yes, task->signal is like sighand: shared across all threads.
--
Kees Cook
Chrome OS Security
next prev parent reply other threads:[~2014-06-25 18:25 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
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 [this message]
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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