linux-mips
[Top] [All Lists]

Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v8 9/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 25 Jun 2014 11:09:40 -0700
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>
In-reply-to: <CAGXu5jKkLS3++_dtWHnjWudVvaSR9DRwjNG3q00SmSy6XoCMaw@mail.gmail.com>
List-archive: <http://www.linux-mips.org/archives/linux-mips/>
List-help: <mailto:ecartis@linux-mips.org?Subject=help>
List-id: linux-mips <linux-mips.eddie.linux-mips.org>
List-owner: <mailto:ralf@linux-mips.org>
List-post: <mailto:linux-mips@linux-mips.org>
List-software: Ecartis version 1.0.0
List-subscribe: <mailto:ecartis@linux-mips.org?subject=subscribe%20linux-mips>
List-unsubscribe: <mailto:ecartis@linux-mips.org?subject=unsubscribe%20linux-mips>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <1403642893-23107-1-git-send-email-keescook@chromium.org> <1403642893-23107-10-git-send-email-keescook@chromium.org> <20140625142121.GD7892@redhat.com> <CAGXu5jJtLrjbobZC1FD4WV-Jm2p7cRGa1aSPK-d_isnfCZAHdA@mail.gmail.com> <20140625165209.GA14720@redhat.com> <CAGXu5jLHDew1fifGY_mWgwcH7evm0T8rqSnBrw4XpoAXGK+t-Q@mail.gmail.com> <20140625172410.GA17133@redhat.com> <CAGXu5jKkLS3++_dtWHnjWudVvaSR9DRwjNG3q00SmSy6XoCMaw@mail.gmail.com>
Sender: linux-mips-bounce@linux-mips.org
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?

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



-- 
Andy Lutomirski
AMA Capital Management, LLC

<Prev in Thread] Current Thread [Next in Thread>