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=-6.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS 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 C9A88C433DF for ; Fri, 21 Aug 2020 10:22:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9CE53207BB for ; Fri, 21 Aug 2020 10:22:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="JH1jyk14" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728675AbgHUKWX (ORCPT ); Fri, 21 Aug 2020 06:22:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33986 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728599AbgHUKWE (ORCPT ); Fri, 21 Aug 2020 06:22:04 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 41A44C061386 for ; Fri, 21 Aug 2020 03:22:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=JyJ51hlk0BrqK7XKW/FKU2cPxtbzAdVRWf6x3vbR6xQ=; b=JH1jyk14xpZ3EtbRbsP2oav/dC 3l/ADogNrquVMrEgvsJk0qEZXF4m2KoTzxJEMalnPjUL3aDy0TlZel9yBeT8EHuRQweH9LvjS22uO pMqUNRGKP+uOJzBo6ox67YdI2anjBameppj6d348u7y58mNaUX6uChzSWSuwrZGuYvUpwBANA5jAE 18x75JNwI9YzkHkQVG2QMABHNnjja+5VnWHI2td8eLKHnsw9jBgeQzC9g1cU4gUmG/JB163C1SxWg GD1FZI1B3pk6NHTiohZsRlApnZjTJ7XI813ozgmeWr5+vKNl0AHYWuYIv0Z/RDezrJ3gmvILjSoB+ F5iSbH5g==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1k94Ao-0003GB-23; Fri, 21 Aug 2020 10:21:26 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id B701F3060F3; Fri, 21 Aug 2020 12:21:24 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 0) id 69E182C8FF36B; Fri, 21 Aug 2020 12:21:24 +0200 (CEST) Message-ID: <20200821102053.219655533@infradead.org> User-Agent: quilt/0.66 Date: Fri, 21 Aug 2020 11:39:20 +0200 From: Peter Zijlstra To: x86@kernel.org Cc: linux-kernel@vger.kernel.org, Kyle Huey , Alexandre Chartre , Robert O'Callahan , "Paul E. McKenney" , Frederic Weisbecker , Paolo Bonzini , Sean Christopherson , Masami Hiramatsu , Petr Mladek , Steven Rostedt , Joel Fernandes , Boris Ostrovsky , Juergen Gross , Brian Gerst , Andy Lutomirski , Josh Poimboeuf , Daniel Thompson , Peter Zijlstra , Andy Lutomirski Subject: [PATCH v2 8/8] x86/debug: Remove the historical junk References: <20200821093912.815135402@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Remove the historical junk and replace it with a WARN and a comment. The problem is that even though the kernel only uses TF single-step in kprobes and KGDB, both of which consume the event before this, QEMU/KVM has bugs in this area that can trigger this state so we have to deal with it. Suggested-by: Brian Gerst Suggested-by: Andy Lutomirski Signed-off-by: Peter Zijlstra (Intel) --- arch/x86/kernel/traps.c | 24 ++++++++++-------------- 1 file changed, 10 insertions(+), 14 deletions(-) --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -839,22 +839,18 @@ static __always_inline void exc_debug_ke goto out; /* - * Reload dr6, the notifier might have changed it. + * The kernel doesn't use TF single-step outside of: + * + * - Kprobes, consumed through kprobe_debug_handler() + * - KGDB, consumed through notify_debug() + * + * So if we get here with DR_STEP set, something is wonky. + * + * A known way to trigger this is through QEMU's GDB stub, + * which leaks #DB into the guest and causes IST recursion. */ - dr6 = current->thread.debugreg6; - - if (WARN_ON_ONCE(dr6 & DR_STEP)) { - /* - * Historical junk that used to handle SYSENTER single-stepping. - * This should be unreachable now. If we survive for a while - * without anyone hitting this warning, we'll turn this into - * an oops. - */ - dr6 &= ~DR_STEP; - set_thread_flag(TIF_SINGLESTEP); + if (WARN_ON_ONCE(current->thread.debugreg6 & DR_STEP)) regs->flags &= ~X86_EFLAGS_TF; - } - out: instrumentation_end(); idtentry_exit_nmi(regs, irq_state);