From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Juergen Gross , Boris Ostrovsky , Thomas Gleixner , David Woodhouse , "Srivatsa S. Bhat" , "Matt Helsley (VMware)" , Alexey Makhalov , Bo Gan Subject: [PATCH 4.4 047/107] x86/amd: dont set X86_BUG_SYSRET_SS_ATTRS when running under Xen Date: Mon, 23 Jul 2018 14:41:41 +0200 Message-Id: <20180723122415.243662310@linuxfoundation.org> In-Reply-To: <20180723122413.003644357@linuxfoundation.org> References: <20180723122413.003644357@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: David Woodhouse commit def9331a12977770cc6132d79f8e6565871e8e38 upstream When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set on AMD cpus. 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 Reviewed-by: Boris Ostrovsky Acked-by: Thomas Gleixner Signed-off-by: Juergen Gross Signed-off-by: David Woodhouse Signed-off-by: Greg Kroah-Hartman Signed-off-by: Srivatsa S. Bhat Reviewed-by: Matt Helsley (VMware) Reviewed-by: Alexey Makhalov Reviewed-by: Bo Gan Signed-off-by: Greg Kroah-Hartman --- arch/x86/kernel/cpu/amd.c | 5 +++-- arch/x86/xen/enlighten.c | 4 +--- 2 files changed, 4 insertions(+), 5 deletions(-) --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -791,8 +791,9 @@ static void init_amd(struct cpuinfo_x86 if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM)) 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); + /* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */ + if (!cpu_has(c, X86_FEATURE_XENPV)) + set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); } #ifdef CONFIG_X86_32 --- a/arch/x86/xen/enlighten.c +++ b/arch/x86/xen/enlighten.c @@ -462,10 +462,8 @@ static void __init xen_init_cpuid_mask(v static void __init xen_init_capabilities(void) { - if (xen_pv_domain()) { - setup_clear_cpu_cap(X86_BUG_SYSRET_SS_ATTRS); + if (xen_pv_domain()) setup_force_cpu_cap(X86_FEATURE_XENPV); - } } static void xen_set_debugreg(int reg, unsigned long val)