From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161046AbbCLRdu (ORCPT ); Thu, 12 Mar 2015 13:33:50 -0400 Received: from mail-vc0-f174.google.com ([209.85.220.174]:59076 "EHLO mail-vc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161017AbbCLRdm (ORCPT ); Thu, 12 Mar 2015 13:33:42 -0400 MIME-Version: 1.0 In-Reply-To: <20150312130701.GA11073@pc.thejh.net> References: <20150311174204.GA5712@pc.thejh.net> <21760.46870.338764.599348@gargle.gargle.HOWL> <20150312130701.GA11073@pc.thejh.net> Date: Thu, 12 Mar 2015 10:33:41 -0700 X-Google-Sender-Auth: VIum0f1WCsZfEbQwUB-wg-ac4d4 Message-ID: Subject: Re: [PATCH] seccomp.2: Add note about alarm(2) not being sufficient to limit runtime From: Kees Cook To: Jann Horn Cc: Michael Kerrisk , Mikael Pettersson , linux-man , Linux API , LKML , Russell King , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , "x86@kernel.org" , Jeff Dike , Richard Weinberger , Andy Lutomirski , Will Drewry Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 12, 2015 at 6:07 AM, Jann Horn wrote: > On Wed, Mar 11, 2015 at 10:43:50PM +0100, Mikael Pettersson wrote: >> Jann Horn writes: >> > Or should I throw this patch away and write a patch >> > for the prctl() manpage instead that documents that >> > being able to call sigreturn() implies being able to >> > effectively call sigprocmask(), at least on some >> > architectures like X86? >> >> Well, that is the semantics of sigreturn(). It is essentially >> setcontext() [which includes the actions of sigprocmask()], but >> with restrictions on parameter placement (at least on x86). >> >> You could introduce some setting to restrict that aspect for >> seccomp processes, but you can't change this for normal processes >> without breaking things. > > Then I think it's probably better and easier to just document the existing > behavior? If a new setting would have to be introduced and developers would > need to be aware of that, it's probably easier to just tell everyone to use > SIGKILL. > > Does this manpage patch look good? > > --- > man2/seccomp.2 | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/man2/seccomp.2 b/man2/seccomp.2 > index 702ceb8..f762d07 100644 > --- a/man2/seccomp.2 > +++ b/man2/seccomp.2 > @@ -64,6 +64,31 @@ Strict secure computing mode is useful for number-crunching > applications that may need to execute untrusted byte code, perhaps > obtained by reading from a pipe or socket. > > +Note that although the calling thread can no longer call > +.BR sigprocmask (2), > +it can use > +.BR sigreturn (2) > +to block all signals apart from > +.BR SIGKILL > +and > +.BR SIGSTOP . > +Therefore, to reliably terminate it, > +.BR SIGKILL > +has to be used, meaning that e.g. > +.BR alarm (2) > +is not sufficient for restricting its runtime. Instead, use > +.BR timer_create (2) > +with > +.BR SIGEV_SIGNAL > +and > +.BR sigev_signo > +set to > +.BR SIGKILL > +or use > +.BR setrlimit (2) > +to set the hard limit for > +.BR RLIMIT_CPU . > + > This operation is available only if the kernel is configured with > .BR CONFIG_SECCOMP > enabled. > -- > 2.1.4 > Thanks! This looks good. Acked-by: Kees Cook -Kees -- Kees Cook Chrome OS Security