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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 19:27:53 +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) {
> +             pid_t failed;
> +
> +             if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
> +                 (thread->seccomp.mode == SECCOMP_MODE_FILTER &&
> +                  is_ancestor(thread->seccomp.filter,
> +                              caller->seccomp.filter)))
> +                     continue;
> +
> +             /* Return the first thread that cannot be synchronized. */
> +             failed = task_pid_vnr(thread);
> +             /* If the pid cannot be resolved, then return -ESRCH */
> +             if (failed == 0)
> +                     failed = -ESRCH;

forgot to mention, task_pid_vnr() can't fail. sighand->siglock is held,
for_each_thread() can't see a thread which has passed unhash_process().

Oleg.


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