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vger.kernel.org via listexpand id S1729771AbgIZAee (ORCPT ); Fri, 25 Sep 2020 20:34:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35076 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729186AbgIZAee (ORCPT ); Fri, 25 Sep 2020 20:34:34 -0400 Received: from mail-pf1-x441.google.com (mail-pf1-x441.google.com [IPv6:2607:f8b0:4864:20::441]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 103EEC0613CE for ; Fri, 25 Sep 2020 17:34:34 -0700 (PDT) Received: by mail-pf1-x441.google.com with SMTP id x123so4828514pfc.7 for ; Fri, 25 Sep 2020 17:34:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amacapital-net.20150623.gappssmtp.com; s=20150623; h=content-transfer-encoding:from:mime-version:subject:date:message-id :references:cc:in-reply-to:to; bh=LQhUSgVwoAzLw92Ygh+sbDkDg9VzTVlbdTb6qu/28UI=; b=0UCfcG1B3lmiWLRmVTEkHEUvtYL/+yhH6MaR4BWOvVAYPKMPznhtEUKI/JMwclvII0 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Skarlatos , Giuseppe Scrivano , Hubertus Franke , Jack Chen , Jann Horn , Josep Torrellas , Tianyin Xu , Tobin Feldman-Fitzthum , Tycho Andersen , Valentin Rothberg , Will Drewry In-Reply-To: <202009251648.4AA27D5B@keescook> To: Kees Cook X-Mailer: iPhone Mail (18A373) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Sep 25, 2020, at 4:49 PM, Kees Cook wrote: >=20 > =EF=BB=BFOn Fri, Sep 25, 2020 at 02:07:46PM -0700, Andy Lutomirski wrote: >>> On Fri, Sep 25, 2020 at 1:37 PM Kees Cook wrote:= >>>=20 >>> On Fri, Sep 25, 2020 at 12:51:20PM -0700, Andy Lutomirski wrote: >>>>=20 >>>>=20 >>>>> On Sep 25, 2020, at 12:42 PM, Kees Cook wrote:= >>>>>=20 >>>>> =EF=BB=BFOn Fri, Sep 25, 2020 at 11:45:05AM -0500, YiFei Zhu wrote: >>>>>> On Thu, Sep 24, 2020 at 10:04 PM YiFei Zhu w= rote: >>>>>>>> Why do the prepare here instead of during attach? (And note that it= >>>>>>>> should not be written to fail.) >>>>>>>=20 >>>>>>> Right. >>>>>>=20 >>>>>> During attach a spinlock (current->sighand->siglock) is held. Do we >>>>>> really want to put the emulator in the "atomic section"? >>>>>=20 >>>>> It's a good point, but I had some other ideas around it that lead to m= e >>>>> a different conclusion. Here's what I've got in my head: >>>>>=20 >>>>> I don't view filter attach (nor the siglock) as fastpath: the lock is >>>>> rarely contested and the "long time" will only be during filter attach= . >>>>>=20 >>>>> When performing filter emulation, all the syscalls that are already >>>>> marked as "must run filter" on the previous filter can be skipped for >>>>> the new filter, since it cannot change the outcome, which makes the >>>>> emulation step faster. >>>>>=20 >>>>> The previous filter's bitmap isn't "stable" until siglock is held. >>>>>=20 >>>>> If we do the emulation step before siglock, we have to always do full >>>>> evaluation of all syscalls, and then merge the bitmap during attach. >>>>> That means all filters ever attached will take maximal time to perform= >>>>> emulation. >>>>>=20 >>>>> I prefer the idea of the emulation step taking advantage of the bitmap= >>>>> optimization, since the kernel spends less time doing work over the li= fe >>>>> of the process tree. It's certainly marginal, but it also lets all the= >>>>> bitmap manipulation stay in one place (as opposed to being split betwe= en >>>>> "prepare" and "attach"). >>>>>=20 >>>>> What do you think? >>>>>=20 >>>>>=20 >>>>=20 >>>> I=E2=80=99m wondering if we should be much much lazier. We could potent= ially wait until someone actually tries to do a given syscall before we try t= o evaluate whether the result is fixed. >>>=20 >>> That seems like we'd need to track yet another bitmap of "did we emulate= >>> this yet?" And it means the filter isn't really "done" until you run >>> another syscall? eeh, I'm not a fan: it scratches at my desire for >>> determinism. ;) Or maybe my implementation imagination is missing >>> something? >>>=20 >>=20 >> We'd need at least three states per syscall: unknown, always-allow, >> and need-to-run-filter. >>=20 >> The downsides are less determinism and a bit of an uglier >> implementation. The upside is that we don't need to loop over all >> syscalls at load -- instead the time that each operation takes is >> independent of the total number of syscalls on the system. And we can >> entirely avoid, say, evaluating the x32 case until the task tries an >> x32 syscall. >>=20 >> I think it's at least worth considering. >=20 > Yeah, worth considering. I do still think the time spent in emulation is > SO small that it doesn't matter running all of the syscalls at attach > time. The filters are tiny and fail quickly if anything "interesting" > start to happen. ;) >=20 There=E2=80=99s a middle ground, too: do it lazily per arch. So we would al= locate and populate the compat bitmap the first time a compat syscall is att= empted and do the same for x32. This may help avoid the annoying extra memor= y usage and 3x startup overhead while retaining full functionality.=