[Xen-devel] [PATCH 3/3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen

[Juergen Gross <jgross@xxxxxxxx>]

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.

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

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 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 cd0abf8ed314..4c2be99fa0fb 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 fb1867fedd29..a11b46f1abbd 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -473,10 +473,8 @@ 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)
-- 
2.13.6


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