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 <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index c375bc672f825ab5134c6f5bdd60572edf700ce0..747f8a20b6291c8e8b69dde4a3a6a9e9d087b3d8 100644 (file)
--- 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 2bea87cc0ff2340c271ebbdb3e81ba0f983640d6..081437b5f3814695cd45eb2f8b2f9cda6aee360d 100644 (file)
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1977,10 +1977,8 @@ 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)