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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C403BC433F5 for ; Mon, 25 Oct 2021 21:28:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9F36660F02 for ; Mon, 25 Oct 2021 21:28:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233488AbhJYVa6 (ORCPT ); Mon, 25 Oct 2021 17:30:58 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:33878 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232689AbhJYVax (ORCPT ); Mon, 25 Oct 2021 17:30:53 -0400 Received: from in01.mta.xmission.com ([166.70.13.51]:54352) by out02.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mf7W9-00AqF5-U6; Mon, 25 Oct 2021 15:28:29 -0600 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95]:41832 helo=email.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mf7W8-003uop-UG; Mon, 25 Oct 2021 15:28:29 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Linus Torvalds Cc: Andy Lutomirski , Linux Kernel Mailing List , linux-arch , Oleg Nesterov , Al Viro , Kees Cook , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "the arch\/x86 maintainers" , H Peter Anvin References: <87y26nmwkb.fsf@disp2133> <20211020174406.17889-10-ebiederm@xmission.com> <875ytkygfj.fsf_-_@disp2133> Date: Mon, 25 Oct 2021 16:28:22 -0500 In-Reply-To: (Linus Torvalds's message of "Mon, 25 Oct 2021 14:12:03 -0700") Message-ID: <87k0i0x095.fsf@disp2133> 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=1mf7W8-003uop-UG;;;mid=<87k0i0x095.fsf@disp2133>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/DScZg9xLaFM+G28oBA4C2vv6AcTHqeoQ= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v2 10/32] signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved. X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org Linus Torvalds writes: > On Mon, Oct 25, 2021 at 1:54 PM Eric W. Biederman wrote: >> >> Update save_v86_state to always complete all of it's work except >> possibly some of the copies to userspace even if save_v86_state takes >> a fault. This ensures that the kernel is always in a sane state, even >> if userspace has done something silly. > > Well, honestly, with this change, you might as well replace the > force_sigsegv() with just a plain "force_sig()", and make it something > the process can catch. The trouble is I don't think there is enough information made available for user space to do anything with the SIGSEGV. My memory is that applications like dosemu very much have a SIGSEGV handler. So I think if it ever happened it could be quite confusing. Not to mention the pr_alert message. But I guess if a test is written like you suggest we can include enough information for someone to make sense of things. > The only thing that "force_sigsgv()" does is to make SIGSEGV > uncatchable. In contrast, a plain "force_sig()" just means that it > can't be ignored - but it can be caught, and it is fatal only when not > caught. > > And with the "always complete the non-vm86 state restore" part change, > there's really no reason for it to not be caught. > > Of course, the other case (where we have no state information for the > "enter vm86 mode" case) is still fatal, and is a "this should never > happen". But the "cannot write to the vm86 save state" thing isn't > technically fatal. > > It should even be possible to write a test for it: passing a read-only > pointer to the vm86() system call. The vm86 entry will work (because > it only reads the vm86 state from it), but then at vm86 exit, writing > the state back will fail. > > Anybody? I am enthusiastic about writing a test, but I will plod in that direction just so I can get this sorted out. Eric