From: Kees Cook <keescook@chromium.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
"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 08:08:11 -0700 [thread overview]
Message-ID: <CAGXu5jJtLrjbobZC1FD4WV-Jm2p7cRGa1aSPK-d_isnfCZAHdA@mail.gmail.com> (raw)
In-Reply-To: <20140625142121.GD7892@redhat.com>
On Wed, Jun 25, 2014 at 7:21 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 06/24, Kees Cook wrote:
>>
>> +static void seccomp_sync_threads(void)
>> +{
>> + struct task_struct *thread, *caller;
>> +
>> + BUG_ON(!spin_is_locked(¤t->sighand->siglock));
>> +
>> + /* Synchronize all threads. */
>> + caller = current;
>> + for_each_thread(caller, thread) {
>> + /* Get a task reference for the new leaf node. */
>> + get_seccomp_filter(caller);
>> + /*
>> + * Drop the task reference to the shared ancestor since
>> + * current's path will hold a reference. (This also
>> + * allows a put before the assignment.)
>> + */
>> + put_seccomp_filter(thread);
>> + thread->seccomp.filter = caller->seccomp.filter;
>> + /* Opt the other thread into seccomp if needed.
>> + * As threads are considered to be trust-realm
>> + * equivalent (see ptrace_may_access), it is safe to
>> + * allow one thread to transition the other.
>> + */
>> + if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
>> + /*
>> + * Don't let an unprivileged task work around
>> + * the no_new_privs restriction by creating
>> + * a thread that sets it up, enters seccomp,
>> + * then dies.
>> + */
>> + if (task_no_new_privs(caller))
>> + task_set_no_new_privs(thread);
>> +
>> + seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
>> + }
>> + }
>> +}
>
> OK, personally I think this all make sense. I even think that perhaps
> SECCOMP_FILTER_FLAG_TSYNC should allow filter == NULL, a thread might
> want to "sync" without adding the new filter, but this is minor/offtopic.
>
> But. Doesn't this change add a new security hole?
>
> Obviously, we should not allow to install a filter and then (say) exec
> a suid binary, that is why we have no_new_privs/LSM_UNSAFE_NO_NEW_PRIVS.
>
> But what if "thread->seccomp.filter = caller->seccomp.filter" races with
> any user of task_no_new_privs() ? Say, suppose this thread has already
> passed check_unsafe_exec/etc and it is going to exec the suid binary?
Oh, ew. Yeah. It looks like there's a cred lock to be held to combat this?
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?
-Kees
--
Kees Cook
Chrome OS Security
next prev parent reply other threads:[~2014-06-25 15:08 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 [this message]
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
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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