[PATCH 4.9 17/61] x86/amd: dont set X86_BUG_SYSRET_SS_ATTRS when running under Xen

From: Greg Kroah-Hartman
Date: Tue Jun 05 2018 - 13:05:30 EST

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


From: Juergen Gross <jgross@xxxxxxxx>

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@xxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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
@@ -857,8 +857,9 @@ 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
@@ -474,10 +474,8 @@ 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())
- }

static void xen_set_debugreg(int reg, unsigned long val)