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Date:	Thu, 22 May 2014 16:11:06 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	James Morris <james.l.morris@...cle.com>,
	Eric Paris <eparis@...hat.com>,
	Juri Lelli <juri.lelli@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Alex Thorlton <athorlton@....com>,
	Rik van Riel <riel@...hat.com>,
	Daeseok Youn <daeseok.youn@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dario Faggioli <raistlin@...ux.it>,
	Rashika Kheria <rashika.kheria@...il.com>,
	liguang <lig.fnst@...fujitsu.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v5 6/6] seccomp: add SECCOMP_EXT_ACT_TSYNC and SECCOMP_FILTER_TSYNC

On Thu, May 22, 2014 at 4:05 PM, Kees Cook <keescook@...omium.org> wrote:
> Applying restrictive seccomp filter programs to large or diverse
> codebases often requires handling threads which may be started early in
> the process lifetime (e.g., by code that is linked in). While it is
> possible to apply permissive programs prior to process start up, it is
> difficult to further restrict the kernel ABI to those threads after that
> point.
>
> This change adds a new seccomp extension action for synchronizing thread
> group seccomp filters and a prctl() for accessing that functionality,
> as well as a flag for SECCOMP_EXT_ACT_FILTER to perform sync at filter
> installation time.
>
> When calling prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT, SECCOMP_EXT_ACT_FILTER,
> flags, filter) with flags containing SECCOMP_FILTER_TSYNC, or when calling
> prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT, SECCOMP_EXT_ACT_TSYNC, 0, 0), it
> will attempt to synchronize all threads in current's threadgroup to its
> seccomp filter program. This is possible iff all threads are using a filter
> that is an ancestor to the filter current is attempting to synchronize to.
> NULL filters (where the task is running as SECCOMP_MODE_NONE) are also
> treated as ancestors allowing threads to be transitioned into
> SECCOMP_MODE_FILTER. If prctrl(PR_SET_NO_NEW_PRIVS, ...) has been set on the
> calling thread, no_new_privs will be set for all synchronized threads too.
> On success, 0 is returned. On failure, the pid of one of the failing threads
> will be returned, with as many filters installed as possible.

Is there a use case for adding a filter and synchronizing filters
being separate operations?  If not, I think this would be easier to
understand and to use if there was just a single operation.

If you did that, you'd have to decide whether to continue requiring
that all the other threads have a filter that's an ancestor of the
current thread's filter.

--Andy
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