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From 7a5ded1f7c88ae162c525486f4be8030fa54fa26 Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@suse.com>
Date: Wed, 30 May 2018 13:09:58 +0200
Subject: [PATCH 93/93] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running
 under Xen

Upstream commit: def9331a12977770cc6132d79f8e6565871e8e38 ("x86/amd:
don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen")

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 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 cd0abf8..4c2be99 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -857,8 +857,9 @@ 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 01120da..fc63c84 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -472,10 +472,8 @@ static void __init xen_init_cpuid_mask(void)
 
 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)
-- 
2.7.4