[PATCH 4.4 047/107] x86/amd: dont set X86_BUG_SYSRET_SS_ATTRS when running under Xen

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

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@xxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Srivatsa S. Bhat <srivatsa@xxxxxxxxxxxxx>
Reviewed-by: Matt Helsley (VMware) <matt.helsley@xxxxxxxxx>
Reviewed-by: Alexey Makhalov <amakhalov@xxxxxxxxxx>
Reviewed-by: Bo Gan <ganb@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---

 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)