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Re: [PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v7 7/9] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC
From: Oleg Nesterov <oleg@redhat.com>
Date: Tue, 24 Jun 2014 19:08:00 +0200
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>, Alexei Starovoitov <ast@plumgrid.com>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.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 06/23, Kees Cook wrote:
>
> +static pid_t seccomp_can_sync_threads(void)
> +{
> +     struct task_struct *thread, *caller;
> +
> +     BUG_ON(write_can_lock(&tasklist_lock));
> +     BUG_ON(!spin_is_locked(&current->sighand->siglock));
> +
> +     if (current->seccomp.mode != SECCOMP_MODE_FILTER)
> +             return -EACCES;
> +
> +     /* Validate all threads being eligible for synchronization. */
> +     thread = caller = current;
> +     for_each_thread(caller, thread) {

You only need to initialize "caller" for for_each_thread(). Same for
seccomp_sync_threads().

> @@ -586,6 +701,17 @@ static long seccomp_set_mode_filter(unsigned int flags,
>       if (IS_ERR(prepared))
>               return PTR_ERR(prepared);
>
> +     /*
> +      * If we're doing thread sync, we must hold tasklist_lock
> +      * to make sure seccomp filter changes are stable on threads
> +      * entering or leaving the task list. And we must take it
> +      * before the sighand lock to avoid deadlocking.
> +      */
> +     if (flags & SECCOMP_FILTER_FLAG_TSYNC)
> +             write_lock_irqsave(&tasklist_lock, taskflags);
> +     else
> +             __acquire(&tasklist_lock); /* keep sparse happy */
> +

Why? ->siglock should be enough, it seems.

It obviously does not protect the global process list, but *sync_threads()
only care about current's thread group list, no?

Oleg.


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