From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1952331AbdDYSAa (ORCPT ); Tue, 25 Apr 2017 14:00:30 -0400 Received: from mx2.suse.de ([195.135.220.15]:40696 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1952316AbdDYSAV (ORCPT ); Tue, 25 Apr 2017 14:00:21 -0400 From: Juergen Gross To: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org Cc: boris.ostrovsky@oracle.com, hpa@zytor.com, tglx@linutronix.de, mingo@redhat.com, Juergen Gross Subject: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero Date: Tue, 25 Apr 2017 20:00:14 +0200 Message-Id: <20170425180014.7533-1-jgross@suse.com> X-Mailer: git-send-email 2.12.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set on AMD cpus. Xen will disable this via setup_clear_cpu_cap(), so test cpu_caps_cleared to not have disabled this bit. This bug/feature bit is kind of special as it will be used very early when switching threads. Setting the bit and clearing it a little bit later leaves a critical window where things can go wrong. This time window has enlarged a little bit by using setup_clear_cpu_cap() instead of the hypervisor's set_cpu_features callback. It seems this larger window now makes it rather easy to hit the problem. The proper solution is to never set the bit in case of Xen. Signed-off-by: Juergen Gross --- arch/x86/kernel/cpu/amd.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index c36140d788fe..f659b6f534b7 100644 --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -800,7 +800,9 @@ static void init_amd(struct cpuinfo_x86 *c) set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH); /* AMD CPUs don't reset SS attributes on SYSRET */ - set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); + if (!test_bit(X86_BUG_SYSRET_SS_ATTRS, + (unsigned long *)cpu_caps_cleared)) + set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); } #ifdef CONFIG_X86_32 -- 2.12.0