From 5fb8da20577a159d311db9c29e62dbb782529571 Mon Sep 17 00:00:00 2001 From: David Woodhouse Date: Sun, 20 May 2018 20:51:10 +0100 Subject: [PATCH 21/93] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen 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 --- arch/x86/kernel/cpu/amd.c | 5 +++-- arch/x86/xen/enlighten.c | 4 +--- 2 files changed, 4 insertions(+), 5 deletions(-) diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index c375bc6..747f8a2 100644 --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -824,8 +824,9 @@ static void init_amd(struct cpuinfo_x86 *c) 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 diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c index 5226379..8b97c87 100644 --- a/arch/x86/xen/enlighten.c +++ b/arch/x86/xen/enlighten.c @@ -1968,10 +1968,8 @@ EXPORT_SYMBOL_GPL(xen_hvm_need_lapic); static void xen_set_cpu_features(struct cpuinfo_x86 *c) { - if (xen_pv_domain()) { - clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); + if (xen_pv_domain()) set_cpu_cap(c, X86_FEATURE_XENPV); - } } static void xen_pin_vcpu(int cpu) -- 2.7.4