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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 8B5D5C5517A for ; Wed, 11 Nov 2020 20:15:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2BFED208B8 for ; Wed, 11 Nov 2020 20:15:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726359AbgKKUPe (ORCPT ); Wed, 11 Nov 2020 15:15:34 -0500 Received: from out03.mta.xmission.com ([166.70.13.233]:40584 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726203AbgKKUPe (ORCPT ); Wed, 11 Nov 2020 15:15:34 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out03.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kcwWe-006g7P-Aw; Wed, 11 Nov 2020 13:15:28 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1kcwWc-0000EG-Pi; Wed, 11 Nov 2020 13:15:28 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Dave Martin Cc: Peter Collingbourne , Catalin Marinas , Evgenii Stepanov , Kostya Serebryany , Vincenzo Frascino , Will Deacon , Oleg Nesterov , "James E.J. Bottomley" , Linux ARM , Kevin Brodsky , Andrey Konovalov , Richard Henderson , linux-api@vger.kernel.org, Helge Deller , David Spickett References: <0eb601a5d1906fadd7099149eb605181911cfc04.1604523707.git.pcc@google.com> <87zh3qug6q.fsf@x220.int.ebiederm.org> <20201111172703.GP6882@arm.com> Date: Wed, 11 Nov 2020 14:15:15 -0600 In-Reply-To: <20201111172703.GP6882@arm.com> (Dave Martin's message of "Wed, 11 Nov 2020 17:27:04 +0000") Message-ID: <87imabr6p8.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1kcwWc-0000EG-Pi;;;mid=<87imabr6p8.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/7N/eWBv/4p/S3APPKai60j3a29t5pU/E= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v14 7/8] signal: define the field siginfo.si_faultflags X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org Dave Martin writes: > On Mon, Nov 09, 2020 at 07:57:33PM -0600, Eric W. Biederman wrote: >> Peter Collingbourne writes: >> >> > This field will contain flags that may be used by signal handlers to >> > determine whether other fields in the _sigfault portion of siginfo are >> > valid. An example use case is the following patch, which introduces >> > the si_addr_tag_bits{,_mask} fields. >> > >> > A new sigcontext flag, SA_FAULTFLAGS, is introduced in order to allow >> > a signal handler to require the kernel to set the field (but note >> > that the field will be set anyway if the kernel supports the flag, >> > regardless of its value). In combination with the previous patches, >> > this allows a userspace program to determine whether the kernel will >> > set the field. >> > >> > It is possible for an si_faultflags-unaware program to cause a signal >> > handler in an si_faultflags-aware program to be called with a provided >> > siginfo data structure by using one of the following syscalls: >> > >> > - ptrace(PTRACE_SETSIGINFO) >> > - pidfd_send_signal >> > - rt_sigqueueinfo >> > - rt_tgsigqueueinfo >> > >> > So we need to prevent the si_faultflags-unaware program from causing an >> > uninitialized read of si_faultflags in the si_faultflags-aware program when >> > it uses one of these syscalls. >> > >> > The last three cases can be handled by observing that each of these >> > syscalls fails if si_code >= 0. We also observe that kill(2) and >> > tgkill(2) may be used to send a signal where si_code == 0 (SI_USER), >> > so we define si_faultflags to only be valid if si_code > 0. >> > >> > There is no such check on si_code in ptrace(PTRACE_SETSIGINFO), so >> > we make ptrace(PTRACE_SETSIGINFO) clear the si_faultflags field if it >> > detects that the signal would use the _sigfault layout, and introduce >> > a new ptrace request type, PTRACE_SETSIGINFO2, that a si_faultflags-aware >> > program may use to opt out of this behavior. >> >> So I think while well intentioned this is misguided. >> >> gdb and the like may use this but I expect the primary user is CRIU >> which simply reads the signal out of one process saves it on disk >> and then restores the signal as read into the new process (possibly >> on a different machine). >> >> At least for the CRIU usage PTRACE_SETSIGINFO need to remain a raw >> pass through kind of operation. > > This is a problem, though. > > How can we tell the difference between a siginfo that was generated by > the kernel and a siginfo that was generated (or altered) by a non-xflags > aware userspace? > > Short of revving the whole API, I don't see a simple solution to this. Unlike receiving a signal. We do know that userspace old and new always sends unused fields as zero into PTRACE_SETSIGINFO. The split into kernel_siginfo verifies this and fails userspace if it does something different. No problems have been reported. So in the case of xflags a non-xflags aware userspace would either pass the siginfo from through from somewhere else (such as PTRACE_GETSIGINFO), or it would simply generate a signal with all of the xflags bits clear. So everything should work regardless. > Although a bit of a hack, could we include some kind of checksum in the > siginfo? If the checksum matches during PTRACE_SETSIGINFO, we could > accept the whole thing; xflags included. Otherwise, we could silently > drop non-self-describing extensions. > > If we only need to generate the checksum when PTRACE_GETSIGINFO is > called then it might be feasible to use a strong hash; otherwise, this > mechanism will be far from bulletproof. > > A hash has the advantage that we don't need any other information > to validate it beyond a salt: if the hash matches, it's self- > validating. We could also package other data with it to describe the > presence of extensions, but relying on this for regular sigaction()/ > signal delivery use feels too high-overhead. > > For debuggers, I suspect that PTRACE_SETSIGINFO2 is still useful: > userspace callers that want to write an extension field that they > knowingly generated themselves should have a way to express that. > > Thoughts? I think there are two cases: 1) CRIU -- It is just a passthrough of PTRACE_GETSIGINFO 2) Creating a signal from nowhere -- Code that does not know about xflags would leave xflags at 0 so no problem. Does anyone see any other cases I am missing? Eric