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 A2807C433EF for ; Fri, 12 Nov 2021 15:40:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 835D661056 for ; Fri, 12 Nov 2021 15:40:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235408AbhKLPnZ (ORCPT ); Fri, 12 Nov 2021 10:43:25 -0500 Received: from out01.mta.xmission.com ([166.70.13.231]:41674 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234508AbhKLPnE (ORCPT ); Fri, 12 Nov 2021 10:43:04 -0500 Received: from in02.mta.xmission.com ([166.70.13.52]:39966) by out01.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mlYex-00HRjL-OZ; Fri, 12 Nov 2021 08:40:11 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95]:60472 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1mlYev-00CI2X-6y; Fri, 12 Nov 2021 08:40:11 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: linux-kernel@vger.kernel.org Cc: Linus Torvalds , Oleg Nesterov , Al Viro , Kees Cook , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, H Peter Anvin , "Andy Lutomirski" References: <87y26nmwkb.fsf@disp2133> <20211020174406.17889-9-ebiederm@xmission.com> Date: Fri, 12 Nov 2021 09:40:01 -0600 In-Reply-To: <20211020174406.17889-9-ebiederm@xmission.com> (Eric W. Biederman's message of "Wed, 20 Oct 2021 12:43:55 -0500") Message-ID: <874k8htmb2.fsf@email.froward.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=1mlYev-00CI2X-6y;;;mid=<874k8htmb2.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/KsN+DLpDCN8Kpyddi7rOkerSB6iMUT1U= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 09/20] signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org "Eric W. Biederman" writes: > The function save_v86_state is only called when userspace was > operating in vm86 mode before entering the kernel. Not having vm86 > state in the task_struct should never happen. So transform the hand > rolled BUG_ON into an actual BUG_ON to make it clear what is > happening. Now that this change has been merged into Linus' tree I have a report that it is possible to trigger this new BUG_ON. Which obviously is not good. We could revert the change but I think that would just be shooting the messenger. Does anyone have an idea where to start to track down what is going on? A very quick skim through the code suggests that the only code path that calls save_v86_state that has not already accessed is current->thread.vm86 is handle_signal. Another quick look suggests that the only place where X86_VM_MASK gets set in eflags is in do_sys_vm86. So it appears do_sys_vm86 must be called before v8086_mode returns true in handle_signal. Which seems to suggest that the bug on can't trigger. But that is obviously wrong. I will keep digging but if anyone has some ideas that would be appreciated. Eric > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: Borislav Petkov > Cc: x86@kernel.org > Cc: H Peter Anvin > Signed-off-by: "Eric W. Biederman" > --- > arch/x86/kernel/vm86_32.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c > index e5a7a10a0164..63486da77272 100644 > --- a/arch/x86/kernel/vm86_32.c > +++ b/arch/x86/kernel/vm86_32.c > @@ -106,10 +106,8 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval) > */ > local_irq_enable(); > > - if (!vm86 || !vm86->user_vm86) { > - pr_alert("no user_vm86: BAD\n"); > - do_exit(SIGSEGV); > - } > + BUG_ON(!vm86 || !vm86->user_vm86); > + > set_flags(regs->pt.flags, VEFLAGS, X86_EFLAGS_VIF | vm86->veflags_mask); > user = vm86->user_vm86;