From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A0FD9C432C0 for ; Fri, 29 Nov 2019 18:59:45 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 68EAE21721 for ; Fri, 29 Nov 2019 18:59:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="bsjFeMuQ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 68EAE21721 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:34446 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ialUU-00049a-Kp for qemu-devel@archiver.kernel.org; Fri, 29 Nov 2019 13:59:42 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:34826) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ialPg-0002iF-UD for qemu-devel@nongnu.org; Fri, 29 Nov 2019 13:54:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ialP5-0002Gr-Pc for qemu-devel@nongnu.org; Fri, 29 Nov 2019 13:54:09 -0500 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:58544 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ialP5-00028X-Gt for qemu-devel@nongnu.org; Fri, 29 Nov 2019 13:54:07 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1575053646; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=GM/hza6bN8BbX6DuTaJLZyYJODxj/MiNEx50kxizkuw=; b=bsjFeMuQ+Y048aLnHQLcxlZFYNjgRi8a9OgPiA3neavbEkFH6aefOXwKwD7L6twtYOogVb 9Pod/fNlEnUUu+96yXyZHUOExSxsmWL9ZO9GJtUyDUndJgu+x0b3MUBSbMZbyJiqOlACA3 rYsJhxULkE1Qv7LauYUHdoyStB7Qbdo= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-218-3MB2UtGgOouQ_kAvBOpKZQ-1; Fri, 29 Nov 2019 13:54:05 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7615EDB21 for ; Fri, 29 Nov 2019 18:54:04 +0000 (UTC) Received: from work-vm (ovpn-117-253.ams2.redhat.com [10.36.117.253]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 50E235D6D2; Fri, 29 Nov 2019 18:54:03 +0000 (UTC) Date: Fri, 29 Nov 2019 18:54:00 +0000 From: "Dr. David Alan Gilbert" To: Paolo Bonzini Subject: Re: libcap vs libcap-ng mess Message-ID: <20191129185400.GF2837@work-vm> References: <20191128190408.GC3294@work-vm> <20191129093409.GB2260471@redhat.com> <98520a07-cf5d-a2a9-cfa4-944839b94c7c@redhat.com> <20191129180103.GA2840@work-vm> <1e59f880-1a62-3230-c56a-533f2b797525@redhat.com> <20191129182021.GD2837@work-vm> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: 3MB2UtGgOouQ_kAvBOpKZQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: thuth@redhat.com, Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , qemu-devel@nongnu.org, vgoyal@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Paolo Bonzini (pbonzini@redhat.com) wrote: > On 29/11/19 19:20, Dr. David Alan Gilbert wrote: > > * Paolo Bonzini (pbonzini@redhat.com) wrote: > >> On 29/11/19 19:01, Dr. David Alan Gilbert wrote: > >>>> It's not entirely trivial because fsdev-proxy-helper wants to keep t= he > >>>> effective set and clear the permitted set; in libcap-ng you can only > >> ^^^^^ > >> > >> (Wrong, this is "modify" the permitted set. The permitted set is > >> already cleared by setresuid/setresgid). > >> > >>>> apply both sets at once, and you cannot choose only one of them in > >>>> capng_clear/capng_get_caps_process. But it's doable, I'll take a lo= ok. > >>> I'm having some difficulties making the same conversion for virtiofsd= ; > >>> all it wants to do is drop (and later recover) CAP_FSETID > >>> from it's effective set; so I'm calling capng_get_caps_process > >>> (it used to be cap_get_proc). While libcap survives just using the > >>> capget syscall, libcap-ng wants to read /proc//status - and > >>> that's a problem because we're in a sandbox without /proc mounted > >>> at that point. > >> > >> The state of libcap-ng persists after capng_apply. So you can just ca= ll > >> capng_update({CAP_ADD,CAP_DROP}) followed by capng_apply. > >=20 > > But the internal state needs initialising doesn't it? So that when you > > capng_update it tweaks a set that was originally read from somewhere? > > (and that's per-thread?) >=20 > Yes, it's per thread. The state can be built from > capng_clear/capng_get_caps_process + capng_update, and left in there > forever. There is also capng_save_state/capng_restore_state which, as > far as I can see from the sources, can be used across threads. OK that's a lot more complex than the current code, and a bit fragile - but probably more efficient. So, I think what you're saying is I need to: a) Before we sandbox do the capng_get_caps_process b) Before we start a new thread do a capng_save_state and restore it in the thread I've got to be pretty careful that I do (a) at the write point so I've not gained anything we later try and drop. (But we do save doing the capget() on every time we do this drop/restore dance). > >> Does virtiofsd have to do uid/gid dances like virtfs-proxy-helper? > >=20 > > It looks like it; I can see setresuid calls to save and restore > > euid/egid. >=20 > Ok, then perhaps you can take a look at my virtfs-proxy-helper patch. > The important part is that after setresuid/setresgid PERM=3DEFF if > uid=3D0/gid=3D0 and PERM=3D0 otherwise. I think we're ok because: a) This code is very local - it does a drop FSETID, a write, restore FSETID b) I'm not sure but I suspect it's used only in the non-uid=3D0 case;=20 the whole thing is just a hack to cause setuid/setgid to be dropped in the case where it's written by a process that doesn't have FSETID (hmm I guess if the guest was root but didn't have fsetid then it would be 0?) But are you suggesting I need to change something other than the effective caps in that case? Dave > Paolo >=20 -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK