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=-5.3 required=3.0 tests=BAYES_00, 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 3124DC388F9 for ; Wed, 11 Nov 2020 17:27:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AF7D22074B for ; Wed, 11 Nov 2020 17:27:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726960AbgKKR1L (ORCPT ); Wed, 11 Nov 2020 12:27:11 -0500 Received: from foss.arm.com ([217.140.110.172]:58658 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726318AbgKKR1L (ORCPT ); Wed, 11 Nov 2020 12:27:11 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0FD3A30E; Wed, 11 Nov 2020 09:27:10 -0800 (PST) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0F7073F6CF; Wed, 11 Nov 2020 09:27:07 -0800 (PST) Date: Wed, 11 Nov 2020 17:27:04 +0000 From: Dave Martin To: "Eric W. Biederman" 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 Subject: Re: [PATCH v14 7/8] signal: define the field siginfo.si_faultflags Message-ID: <20201111172703.GP6882@arm.com> References: <0eb601a5d1906fadd7099149eb605181911cfc04.1604523707.git.pcc@google.com> <87zh3qug6q.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87zh3qug6q.fsf@x220.int.ebiederm.org> User-Agent: Mutt/1.5.23 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org 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. 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? Cheers ---Dave