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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: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 25 Jun 2014 16:21:21 +0200
Cc: 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@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org
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On 06/24, Kees Cook wrote:
>
> +static void seccomp_sync_threads(void)
> +{
> +     struct task_struct *thread, *caller;
> +
> +     BUG_ON(!spin_is_locked(&current->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 the 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?

Oleg.


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