From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Dmitry V. Levin" Subject: Re: [PATCH v5 3/5] x86: Split syscall_trace_enter into two phases Date: Fri, 6 Feb 2015 02:39:45 +0300 Message-ID: <20150205233945.GA31540@altlinux.org> References: <2df320a600020fda055fccf2b668145729dd0c04.1409954077.git.luto@amacapital.net> <20150205211916.GA31367@altlinux.org> <20150205214027.GB31367@altlinux.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from pegasus3.altlinux.org ([194.107.17.103]:44757 "EHLO pegasus3.altlinux.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753470AbbBEXjs (ORCPT ); Thu, 5 Feb 2015 18:39:48 -0500 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Kees Cook Cc: Andy Lutomirski , LKML , Will Drewry , Oleg Nesterov , "x86@kernel.org" , "linux-arm-kernel@lists.infradead.org" , Linux MIPS Mailing List , linux-arch , linux-security-module , Alexei Starovoitov , "H. Peter Anvin" , Frederic Weisbecker On Thu, Feb 05, 2015 at 03:12:39PM -0800, Kees Cook wrote: > On Thu, Feb 5, 2015 at 1:52 PM, Andy Lutomirski wrote: > > On Thu, Feb 5, 2015 at 1:40 PM, Dmitry V. Levin wrote: > >> On Thu, Feb 05, 2015 at 01:27:16PM -0800, Kees Cook wrote: > >>> On Thu, Feb 5, 2015 at 1:19 PM, Dmitry V. Levin wrote: > >>> > Hi, > >>> > > >>> > On Fri, Sep 05, 2014 at 03:13:54PM -0700, Andy Lutomirski wrote: > >>> >> This splits syscall_trace_enter into syscall_trace_enter_phase1 and > >>> >> syscall_trace_enter_phase2. Only phase 2 has full pt_regs, and only > >>> >> phase 2 is permitted to modify any of pt_regs except for orig_ax. > >>> > > >>> > This breaks ptrace, see below. > >>> > [...] > >>> >> + ret = seccomp_phase1(&sd); > >>> >> + if (ret == SECCOMP_PHASE1_SKIP) { > >>> >> + regs->orig_ax = -1; > >>> > > >>> > How the tracer is expected to get the correct syscall number after that? > >>> > >>> There shouldn't be a tracer if a skip is encountered. (A seccomp skip > >>> would skip ptrace.) This behavior hasn't changed, but maybe I don't > >>> see what you mean? (I haven't encountered any problems with syscall > >>> tracing as a result of these changes.) > >> > >> SECCOMP_RET_ERRNO leads to SECCOMP_PHASE1_SKIP, and if there is a tracer, > >> it will get -1 as a syscall number. > >> > >> I've found this while testing a strace parser for > >> SECCOMP_MODE_FILTER/SECCOMP_SET_MODE_FILTER, so the problem is quite real. > > > > Hasn't it always been this way? > > As far as I know, yes, it's always been this way. The point is to the > skip the syscall, which is what the -1 signals. Userspace then reads > back the errno. There is a clear difference: before these changes, SECCOMP_RET_ERRNO used to keep the syscall number unchanged and suppress syscall-exit-stop event, which was awful because userspace cannot distinguish syscall-enter-stop from syscall-exit-stop and therefore relies on the kernel that syscall-enter-stop is followed by syscall-exit-stop (or tracee's death, etc.). After these changes, SECCOMP_RET_ERRNO no longer causes syscall-exit-stop events to be suppressed, but now the syscall number is lost. -- ldv